Luteal Research Articles

P-659 Letrozole versus hormone replacement therapy for endometrial preparation in frozen-thawed embryo transfer cycles: a systematic review and meta-analysis

Abstract Study question Is the probability of pregnancy different when letrozole as compared with hormone replacement therapy (HRT) is used for endometrial preparation in frozen-thawed embryo transfer(FRET) cycles? Summary answer Α significantly higher probability of pregnancy is present when letrozole as compared with HRT is used for endometrial preparation in FRET cycles. What is known already Endometrial preparation with HRT has been a popular treatment during FRET cycles, mainly due to its convenience in cycle programming and its efficacy regarding pregnancy rates. However, recent data, focusing on obstetric outcomes after FRET, have highlighted the importance of corpus luteum (CL), favouring the adoption of strategies that lead to CL development. This is feasible either using a natural cycle or ovarian stimulation with gonadotrophins, clomiphene citrate or letrozole. Although a CL is present in letrozole stimulated cycles, whether the probability of pregnancy is maintained as compared to endometrial preparation with HRT has not yet been sufficiently elucidated. Study design, size, duration A systematic review and meta-analysis was performed aiming to identify randomized clinical trials (RCTs) comparing letrozole versus HRT in women undergoing endometrial preparation for FRET, irrespective of the type of infertility. For this purpose, a literature search was carried out until January 2024 in PubMed, Scopus, EmBase and CENTRAL. The primary outcome measure was live birth rate, while clinical pregnancy rate, ongoing pregnancy rate, endometrial thickness and miscarriage rate were evaluated as secondary outcomes. Participants/materials, setting, methods Citation, demographic, methodological and clinical data were extracted independently by two reviewers. Quality was assessed using the RoB2 Tool by Cochrane. Statistical heterogeneity was assessed by I2. Meta-analysis was performed using the random and fixed effects model depending on the presence or not of significant statistical or clinical heterogeneity. All results were combined using the intention-to-treat principle. Results are reported as relative risk (RR) or weighted mean difference (WMD) with 95% confidence interval (CI). Main results and the role of chance Four RCTs (n = 1242) were identified, published between 2014 and 2023. All of them were deemed as of high risk of bias. The number of patients included in these studies ranged from 163 to 713 patients. Letrozole was administered at doses of 2.5-5mg/day from day 2-3 of the menstrual cycle for 5 days, whereas in HRT cycles, estradiol valerate was administered at doses of 4-8mg/day from day 2-3 of the menstrual cycle. A significantly higher live birth rate was observed using letrozole for endometrial preparation as compared to HRT in women undergoing FRET (RR: 1.34, 95% CI:1.12 to 1.59; fixed effects model; heterogeneity: I2:0%; two studies, 883 patients). Similar differences between letrozole and HRT were also present regarding clinical pregnancy rate (RR: 1.23, 95% CI:1.08 to 1.39; fixed effects model; heterogeneity: I2:0%; four studies, 1240 patients) or ongoing pregnancy rate (RR:1.48, 95% CI:1.04 to 2.12; single study, 190 patients). No significant differences were observed using letrozole for endometrial preparation as compared to HRT regarding either miscarriage rate (RR:0.70, 95% CI:0.46 to 1.07; fixed effects model; heterogeneity: I2:0%; four studies, 1240 patients) or endometrial thickness (WMD:+0.09 mm, 95% CI: -0.86 to + 1.03; random effects model; heterogeneity: I2: 98.1%; three studies, 1070 patients). Limitations, reasons for caution This meta-analysis included a limited number of RCTs, which were generally of high risk of bias due to methodological limitations. Moreover, a considerable heterogeneity was present regarding the dosage of the medications administered and duration of treatment, warranting a cautious interpretation of the results obtained. Wider implications of the findings Endometrial preparation with letrozole seems to be preferable compared to HRT for FRET. In addition to an increased probability of pregnancy observed in the present meta-analysis, letrozole ensures the presence of CL which has been associated with a lower probability of hypertensive disorders of pregnancy. Trial registration number PROSPERO: CRD42024501838

P-449 Association of corpus luteum activity in early pregnancy and live birth after assisted reproductive technology (ART) and intrauterine insemination (IUI)

Abstract Study question What is the incidence of low corpus luteum activity in early pregnancy, and how are endogenous progesterone levels and live birth achievement associated? Summary answer Frequently occurring low corpus luteum activity (progesterone-level) in early pregnancy after ART and IUI strongly predicts pregnancy demise, though not independent of trophoblast activity (hCG-level). What is known already Ovarian stimulation and human chorion gonadotropine (hCG) administration may induce a luteal phase defect via negative feedback on the hypothalamic-pituitary-ovarian (HPO) axis. Luteal phase support (LPS) is routinely administered with bio-identical progesterone or oral dydrogesterone (DYD). DYD has high oral availability, minimal interaction with the pituitary gland, and no cross-reactivity in enzyme-linked immunosorbent assay (ELISA) with progesterone. Therefore, utilizing DYD as LPS in IVF/ICSI or ovulatory IUI cycles provides a unique perspective to assess endocrine luteal activity. While various studies explored progesterone and hCG post-IVF or IUI, none focused solely on the corpus luteum’s sole endocrine function and its outcomes. Study design, size, duration This single-center, retrospective study (with institutional review board approval) investigated hCG and progesterone serum levels after IVF/ICSI cycles or IUI cycles supported with DYD as the sole LPS. The study aims to assess their predictive value, individually and in combination, for sustaining pregnancy. Data from 189 IVF/ICSI and 198 IUI early pregnancies, respectively, with positive hCG test between January 2017 and June 2022, was extracted from the patient management system at the University Fertility Center. Participants/materials, setting, methods Included were only first IVF/ICSI and IUI treatment-cycles with serum hCG value ≥ limit of detection (0.6 IU/l) between days 9 and 16 after fresh embryo transfer or IUI, respectively. Additionally to hCG, endogenous progesterone was determined and the relationship between these and pregnancy maintenance was investigated using logistic regression analysis. Inclusion criteria encompassed hCG-triggered cycles using DYD as the sole LPS, administered at 30 and 20 mg daily for IVF/ICSI and IUI therapy, respectively. Main results and the role of chance The 10th, 25th, 50th and 75th quantiles of serum progesterone are at 0.28, 3.75, 33.0 and 98.7 µg/l in IVF/ICSI cycles and 0.48, 2.70, 18.40 and 26.6 µg/l in IUI cycles. Women in the first quartile of progesterone levels had nearly exclusively pregnancy demise: live birth rate 3/47 (7%, 95% confidence interval (CI) 1-19%) and 0/47 (0%, 95% CI 0-7.5%) in IVF/ICSI and IUI cycles, respectively. In both IVF/ICSI and IUI, progesterone and hCG were highly correlated (correlation coefficient 0.83 in both cases). Accordingly, the association of serum hCG levels with live birth achievement was similar in quartile analyses. Using receiver operating characteristic (ROC) curves and Youden’s indices, the optimal cut-off values for predicting live birth were determined as follows: 16.49 µg/l (89 % sensitivity (SN) and 59 % specificity (SP)) for IVF/ICSI and 15.89 µg/l (90 % SN and 70 % SP) for IUI therapy. Logarithmic regression analysis also revealed a significant correlation between progesterone levels and live birth in IVF/ICSI and IUI therapy, with a p-value of < 0.001. Limitations, reasons for caution Low progesterone in early pregnancy may be a consequence or a cause for pregnancy demise. DYD, like any other progesterone for LPS, might adversely impact corpus luteum formation and maintenance through HPO axis interaction, especially in mono-ovulatory cycles or in older women with inherently higher risk of luteal phase defect. Wider implications of the findings Ovarian stimulation and hCG-administration may induce a luteal phase defect with insufficient luteal activity in early pregnancy, even up to luteolysis, with potential consequences for pregnancy maintenance and maternal/fetal health. Future treatment protocols may require exploring an optimal luteotropic state at pregnancy’s outset, necessitating the establishment of relevant markers. Trial registration number not applicable

P-410 A modified artificial cycle-frozen embryo transfer(mAC-FET) protocol with evidence of corpus luteum(CL) formation as determined by serum relaxin-2 in early pregnancy :a prospective proof-of-concept study

Abstract Study question Can a mAC-FET protocol for endometrial preparation in ovulatory patients lead to formation of a CL as evidenced by serum relaxin-2 in the pregnant patients? Summary answer CL forms in the mAC-FET protocol as serum relaxin-2 in early pregnancy was comparable to that in spontaneous conception.Late luteal rise of relaxin-2 wasn’t demonstrated. What is known already Clinical evidence points to increased incidence of preeclampsia in pregnancies by AC-FET cycles compared to ovulatory FET cycles-natural cycle(NC) and mNC. The absence of CL and it’s secretory products like relaxin-2 is a contributing factor. FET protocols for endometrial preparation in ovulatory patients need to be optimised. NC/mNC-FET protocols need frequent monitoring. Also there is a temporal variability between LH/hCG and progesterone levels. We studied a mAC-FET protocol in ovulatory patients with a delayed start(day3-5 of the cycle),low dose transdermal 17β estradiol gel to see the formation of CL as elucidated by the presence of serum relaxin-2 in early pregnancy. Study design, size, duration A proof-of- concept prospective study was conducted at a tertiary centre (July2023-Dec2023). Serum relaxin-2(R&D/Quantikine/ELISA) was determined in 4 groups of patients- Group-A. mAC-FET(n = 34) 1.On the day of start of progesterone 2.Post-ET day5-9 (along with serum progesterone) 3.In pregnancy(6-9 weeks of gestation) Group-B. AC-FET(n = 8) 1.On the day of start of progesterone 2.In pregnancy(6-9 weeks) Group-C. Spontaneous conception (n = 9) 1.In pregnancy(6-9 weeks) Group-D. Infertility patients ( n = 24) 1.Preovulatory. All patients were ≤40 years with a BMI ≤30kg/m2. Participants/materials, setting, methods In the AC-FET cohort-oral estradiol 6mg/day was started from day1/2 of the cycle. When endometrial thickness was>7mm,progesterone<1ng/ ml then intramuscular progesterone was started- 50mg/day-3 days,100mg on day4-the day of ET(Day-3 embryos) and continued every alternate day.Progesterone gel was added post-ET. In the mAC-FET cohort-17β transdermal estradiol gel(0.06%) 3mg/day was started from day3-5,rest of the protocol was similar. Statistical analysis was performed using appropriate non-parametric tests.Pearson coefficient(r) was used to assess linear-relationship.Two-tailed P<.05 indicated statistical significance. Main results and the role of chance Mean age and BMI of the patients with mAC-FET was 33.3±4.7 years and 24.4±3.7 kg/ m2 which was comparable across all groups (P=.14,P=.52 respectively). Clinical pregnancy rate in the mAC-FET group was 52.94%(18/34). 3 patients had an early abortion. Baseline relaxin-2 levels, median[Interquartile range(IQR)] were comparable- AC-FET cohort-9.1pg/mL(8.6-16.8), mAC-FET cohort-11.5pg/mL(9.6-13.7) and the infertility cohort-10.9pg/mL(8.4-14.5) (P=0.74) The primary outcome was the comparable levels of serum relaxin-2, in early pregnancy in the mAC-FET group and spontaneous conception group - median (IQR) 449.8pg/mL(73.1-492.3) and 487.9pg/mL(419.1-497.8) respectively (P=.12) The relaxin-2 levels, in early pregnancy in the mAC-FET group were significantly higher than the AC-FET group median(IQR) 449.8pg/mL(73.1-492.3) and 7.9pg/mL(7.8-9.5) respectively (P=.0005). As secondary outcomes, we studied the late luteal rise of relaxin-2 in the mAC-FET group and the relationship of late luteal progesterone with relaxin-2 in early pregnancies in the mAC-FET cohort. A late luteal rise of relaxin-2 was not demonstrated in the mAC-FET cohort. The serum relaxin-2, on the day of start of progesterone and on day5-9 post ET- median(IQR) 11.5pg/mL(9.6-13.7) and 11.4pg/mL(9.0-18.5), respectively was comparable (P=.18). The relationship between serum relaxin-2 in early pregnancy and the progesterone levels on day5-9 post ET in the mAC-FET cohort was not significant r = -.059 (P=.07) Limitations, reasons for caution This was a small proof- of-concept single centre study. Due to the limited numbers and short duration of the study, we could not compare efficacy, cancellation rates and pregnancy outcomes of the mAC-FET protocol with other FET protocols. Wider implications of the findings The mAC-FET protocol studied for endometrial preparation was similar to the AC -FET protocol regarding monitoring,flexibility and consistent progesterone exposure with the added advantage of the presence of the CL.This may decrease the risk of maternal/neonatal complications. Larger studies are needed to confirm it’s efficacy and impact on pregnancy outcomes. Trial registration number Not Applicable

P-598 Novel inactivating homozygous mutation of the luteinizing hormone/choriogonadotropin receptor (LHCGR) and first live birth after in vitro maturation (IVM): a breakthrough

Abstract Study question How can pregnancy and live birth be achieved in patients with inactivating homozygous luteinizing hormone/choriogonadotropin receptor (LHCGR) mutation? Summary answer We report the first live birth after IVM in a patient with a novel homozygous inactivating LHCGR mutation (exon 6,c.470A>G). What is known already LH plays a fundamental role in female reproductive physiology and is responsible for steroidogenesis, oocyte maturation, ovulation and subsequent progesterone production by the corpus luteum. LH binds to the LHCGR located on the membrane of theca cells and mature granulosa cells. Inactivating mutations of the LHCG receptor lead to the impossibility to obtain final oocyte maturation both during natural cycles and after ovarian stimulation for in vitro fertilization purpose. Until now, egg donation represented the only option to treat their infertility. Study design, size, duration A case report. Participants/materials, setting, methods A 35 year-old nulliparous patient was referred to our university hospital for primary infertility. Main results and the role of chance The 35-year-old nulliparous patient presented with primary spaniomenorrhea but timely and spontaneous onset of secondary sexual characteristics. Serum LH levels were high (ranging from 15 to 30 IU/L) and to a lesser extent so were FSH levels. The ovarian reserve was normal for age, as assessed by serum AMH levels and ultrasound. There was no argument for polycystic ovarian syndrome, 21-hydroxylase deficiency, Cushing’s syndrome or hyperprolactinemia. Two previous attempts of controlled ovarian stimulation with gonadotropins and ovulation trigger with hCG and GnRH-agonist trigger had failed, resulting in low estradiol levels despite correct follicular growth on ultrasound and absence of ovulation after trigger. We identified by genetic analysis a novel homozygous inactivating LHCGR mutation (exon 6, c.470A>G) that had never been described previously. IVM was performed. A total of 7 oocytes were obtained after IVM, resulting in 4 Day 3 embryos. All embryos were frozen. Subsequently, 2 Day 3 embryos were replaced after endometrial preparation by hormone replacement therapy. The patient became pregnant and gave birth to a healthy baby. Two Day 3 embryos remain. Limitations, reasons for caution By definition, a case-report requires further studies to confirm our findings. Wider implications of the findings We describe a novel inactivating LHCGR mutation with subsequent live birth after IVM. As a growing number of gonadotropin receptor mutations are identified, this live birth successfully obtained places IVM as the only reliable option for these patients to conceive with their own eggs. Trial registration number not applicable

A retrospective analysis of conception per embryo transfer in dairy cattle in South Korea

The bovine embryo production industry has seen significant growth over the past two decades, particularly in the production of in vitro produced embryos. This growth, driven by advancements in cryopreservation, in vitro culture mediums, ovum pick-up (OPU) procedures, ultrasonography devices, and embryo transfer (ET) has been notable. Particularly, ET is crucial for disseminating high genetic merit and amplifying foreign breeds by importing frozen embryos. This retrospective study aimed to assess factors affecting conception per embryo transfer (CPET) in Holstein-Friesian cattle in South Korea from October 2008 to July 2022. We evaluated type of embryo breed, type of embryo production (fresh and frozen; in vitro and in vivo production), recipient conditions including estrus type, corpus luteum quality, parity (nulliparous heifers, primiparous, and multiparous cows), and the daily mean temperature-humidity index (THI) as an index for heat stress. Type of embryo breed and estrus had no significant impact on CPET. However, we observed higher CPET in recipients with good quality corpus luteum, nulliparous heifers, and surrogates receiving fresh in vitro and frozen in vivo embryos. Importantly, CPET was not adversely affected by mild heat stress conditions (up to daily mean THI 76), indicating that using frozen in vivo embryos produced by multiple ovulation embryo transfer and fresh in vitro embryos by OPU-ET can help alleviate the subfertility issues in dairy cattle caused by global warming in Korea.

O-208 The Programmed Ovulatory Frozen-Thawed Embryo Transfer Cycle (PO-FET): a suggested FET protocol integrating aspects of embryo transfer scheduling, efficacy optimization and maternal/fetal safety

Abstract Study question What is the effect of 30mg oral dydrogesterone (DYD), given to induce endometrial receptivity in natural-proliferative-phase-FET-cycles (NPP-FET), on ovulation, progesterone and DYD/DHD-levels, respectively, and outcomes? Summary answer Oral DYD 30mg allows scheduling endometrial receptivity in natural cycles while not interfering with ovulation in the vast majority of patients. What is known already Globally, approximately 60% of all ETs are performed after cryopreservation. Hormone Replacement Treatment (HRT)-FET cycle-regimens have come under scrutiny, because of i) risk of insufficient progesterone exposure with conventional dosing-schemes and ii) maternal/fetal risks stemming from the iatrogenic lack of a corpus luteum. Oral DYD 10mg (tid) supposedly does not interfere with ovulation/corpus luteum formation, and does not cross-react with progesterone in ELISA. It could thus be used for inducing endometrial receptivity (scheduling, in a natural cycle, the time point of ET) and luteal phase support, while allowing ovulation dissociated from the window-of-implantation and monitoring of endogenous progesterone serum levels. Study design, size, duration Within a multi-centric, prospective, clinical cohort study (NCT03507673), 559 normally cycling women from the routine care population who underwent FET after IVF/ICSI in a natural cycle (2/2021 – 8/2023) had serum/plasma samples prospectively collected/stored on day of FET for measurement of E2, progesterone and DYD/DHD in Roche Elecsys®-Immunoassay and liquid chromatography/tandem mass spectroscopy (LC-MS/MS), respectively. FET was performed on day 2/3 (cleavage-stage) or day 4/5 (morula/blastocyst) in 17.4% and 82.6% of FETs, respectively. Participants/materials, setting, methods From cycle day 10, women underwent endocrine (E2, LH and P) and sonographic monitoring until E2 >180pg/ml, leading follicle >16mm and endometrial thickness was >6mm. Physicians could then initiate oral DYD10mg (tid) or–in absence of LH rise–postpone initiation, under 2-daily monitoring, to meet the organizational preferences of the patient and/or center (for example, avoiding weekends). FET was performed for day 2 (cleavage), 3, 4 or 5 (blastocyst) on day 3-6 of DYD-intake, respectively. Main results and the role of chance Previously reported PK of 10mg DYD (tid) are confirmed/extended. While log-levels of DYD and DHD were highly correlated (0.95), P-levels were not (0.02, 0.02). In 94.4% of patients, ovulation could be confirmed by serum-P levels on the day of FET or in late luteal phase. Medians (IQR) of DYD and P on day 3 and days 5, respectively, were 0.93ng/ml (0.93ng/ml) and 2.96ng/ml (3.19ng/ml), and 1.13ng/ml (1.09ng/ml) and 8.50ng/ml (4.40ng/ml). Serum-P levels increased with the FET day (medians 2.0, 3.0, 6.6, and 8.5 ng/ml on days 2, 3, 4, and 5) and only very weakly correlated with LH, E2 levels, and follicular diameter at last monitoring. 42.5% and 34.8% of patients had positive hCG and ongoing clinical pregnancy, respectively. Ongoing pregnancy was independent from plasma-DYD levels (< 25th percentile [<0.66 ng/ml] 37.9% vs. ≥25thperc. [≥0.66 ng/ml] 34.3%, RD 1.5%, 95%CI: -16.5% to 17.4%, p = 0.334), yet dependent from serum-P levels (< 25thperc [<4.85 ng/ml] 25.9% vs. ≥25thperc [≥4.85 ng/ml] 35.3%, RD 9.4%, 95% CI 0.4% to 17.6%, p = 0.0161).However, neither of the two serum hormone values is predictive of a dichotomous outcome in isolation, nor do they demonstrate predictive interaction when considered together in logistic regression modelling. Limitations, reasons for caution DYD, like hCG or any other progestin given for LPS, may negatively interfere with LH surge, ovulation and luteal phase characteristics. Controlled studies on different progestin types and doses are necessary to better understand how to optimize luteal phase and thereby optimally support embryo nidation, pregnancy maintenance and maternal/fetal health. Wider implications of the findings The PO-FET protocol induces the implantation window, thus offers (some) flexibility in FET timing, shows little interference with ovulation/corpus luteum formation, provides double gestagenic support with no need for monitoring P or DYD levels, however allows corpus luteum activity monitoring in luteal phase and early pregnancy, is injection-free, and low-cost. Trial registration number NCT03507673

P-452 A decrease in serum progesterone-levels (P4) during early luteal phase in modified-natural frozen embryo transfer (mNC-FET) is significantly associated with lower pregnancy rates

Abstract Study question Does a serum progesterone-level (P4) increase (+ΔP4) or decrease (-ΔP4) through luteal phase (LP) compromise ongoing pregnancy rate (OPR) or miscarriage rate (MR) in mNC-FET? Summary answer A P4 decrease (-ΔP4) through luteal phase in mNC-FET is significantly associated with lower OPR and higher MR compared to P4 increase (+ΔP4) What is known already Increasing evidence has been recently reported in literature according to the relevance of P4 round FET but also on b-HCG day in artificial and natural FET (AC-FET; NC-FET). Otherwise, mNC-FET gives the chance for triggering and scheduling FET, with less days of monitoring for patients, but keeping the advantages described according to lower risk of preeclampsia through the relaxin production by the corpus luteum. Nevertheless, no studies have been reported according to reproductive outcomes and P4 increase or decrease through the luteal-phase in mNC-FET, as it has been done in fresh embryo transfer (IVF-transfer) Study design, size, duration This is a retrospective observational study led at a single university-affiliated fertility center. Overall, 504 women undergoing mNC-FET between October 2019-October 2023 were included for the analysis. Among them 373 (74%) were homologous-FET cycles (hom-FET), 123 (24%) heterologous-FET cycles (oocyte donation) (het-FET) and 8 (1.6%) embryo donation-FET cycles (don-FET) Participants/materials, setting, methods mNC-FETs were performed seven days after HCG-trigger (HCG+7) and on the sixth day of vaginal progesterone treatment. P4-levels were measured on the day before FET (D5-P4) and on β-HCG day (β-P4). Pregnancy outcomes were analysed according to + ΔP4 or -ΔP4 between those two time intervals through the LP in mNC-FET. Progesterone treatment continued until the 10th week of pregnancy.Chi-squared test was used to compare proportions and ROC-curve analysis to identify the best cutoffs for outcomes Main results and the role of chance Patients’ age at oocyte retrieval was 33.5 and 38.4 at FET. No differences were described according to epidemiological features. Mean β-P4 and D5-P4 (ng/mL) were 25.15 ± 12.8 and 24.88 ± 6.9 respectively. ΔP4 was reported as positive (+ΔP4) in 52.2% (n = 263), and negative (-ΔP4) in 47.8% patients (n = 241). Mean ΔP4 was +0.26 ± 13 ng/mL. Overall reproductive outcomes were: OPR 47.8% and miscarriage rate (MR) 12.3%. OPR was significantly lower (15.8% vs 77.2%, p = 0.00) and MR significantly higher (19.1% vs 11%, p = 0.099) in case of -ΔP4 compared to + ΔP4. Predicting factors for OPR were b-HCG (ROC-AUC (95% CI) 0.920 (0.896-0.945)), β-P4 (ROC-AUC (95% CI) 0.860 (0.827-0.893)) and ΔP4 (ROC-AUC (95% CI) 0.859 (0.826-0.893)). The optimal cut-off value for OPR were β-HCG > 45.15 UI, β-P4 >22.6 ng/mL and ΔP4 > 0.73 ng/mL, with a sensitivity and a specificity of 0.84-0.75, 0.97-0.78 and 0.83-0.78 respectively. OPR in patients with β-HCG > 45.15 UI and β-P4 >22.6 ng/mL was significantly higher (81.7%) compared to patients with β-HCG > 45.15 UI or β-P4 > 22.6 ng/mL (53%), and patients with β-HCG ≤ 45.15 UI and β-P4 ≤ 22.6 ng/mL (2.6%) (p < 0.001) Limitations, reasons for caution The main limitation of this study is its retrospective nature and that conclusions cannot be extrapolated to other methods for endometrial preparation for FET, although conflicting pregnancy outcomes according P4-levels have also been described in either natural or artificial FET. No live birth rate was available at the period study Wider implications of the findings ΔP4 through LP seems to be relevant for pregnancy outcomes in mNC-FET as -ΔP4 significantly reduces OPR and increases MR compared to + ΔP4. Evaluation of ΔP4 between the day before FET (D5-P4) and on β-HCG day (β-P4) may be a useful biomarker to predict ongoing pregnancy rates in mNC-FET cycles Trial registration number Not applicable

O-146 Serum Progesterone levels do not differ between patients with endometriosis and unaffected patients who conceive after Hormone Replacement Therapy-Frozen Embryo Transfer (HRT-FET) cycles

Abstract Study question Do endometriosis women who achieve a live birth (LB) after Hormone Replacement Therapy-Frozen Embryo Transfer (HRT-FET) have different transfer-day progesterone levels than controls? Summary answer In women achieving a LB after HRT-FET, serum progesterone levels on the day of the transfer did not differ between endometriosis and unaffected patients. What is known already In HRT-FET, several studies have highlighted the correlation between serum progesterone levels at the time of frozen embryo transfer and LB rates. In the pathophysiology of endometriosis, progesterone resistance is typically described in the eutopic endometrium. This has led to the hypothesis that women with endometriosis may require higher progesterone levels to achieve a LB, especially in HRT-FET cycles without a corpus luteum. Study design, size, duration We conducted an observational cohort study at the university-based reproductive medicine center of our institution, focusing on women who underwent a single autologous frozen blastocyst transfer after HRT using exogenous estradiol and micronized vaginal progesterone for endometrial preparation between January 2019 and December 2021. Women were included only once during the study period. Serum progesterone levels were measured on the morning of the FET by a single laboratory. Participants/materials, setting, methods Patients were divided into groups based on whether they had endometriosis or not and whether they achieved a LB. The diagnosis of endometriosis was based on published imaging criteria (transvaginal sonography/magnetic resonance imaging) and/or confirmed histology. The primary outcome was progesterone levels on the day of the HRT-FET leading to a LB in patients with endometriosis compared to unaffected women. Subgroup analyses were performed based on the presence of deep infiltrating endometriosis or adenomyosis. Main results and the role of chance A total of 1784 patients were included. The mean age of the women was 35.1 ± 4.1 years. Five hundred and sixty women had endometriosis, while 1224 did not. 179/560 (32.0%) with endometriosis and 381/1224 (31.2%) without endometriosis achieved a LB. Among women who achieved a LB after HRT-FET, there was no significant difference in the mean progesterone level on the day of the HRT-FET between those with endometriosis and those without (13.6 ± 4.3 ng/mL versus 13.2 ± 4.4 ng/mL, respectively; p = 0.302). In the subgroup of women with deep infiltrating endometriosis (n = 142) and adenomyosis (n = 100), the mean progesterone level was 13.1 ± 4.1 ng/mL and 12.6 ± 3.7 ng/mL, respectively, with no significant difference compared to endometriosis-free patients. After adjustment for BMI, parity, duration of infertility, and tobacco use, neither the presence of endometriosis (coefficient 0.38; 95% CI -0.63 to 1.40; p = 0.457) nor the presence of adenomyosis (coefficient 0.97; 95%CI -0.24 to 2.19; p = 0.114) was associated with the progesterone level on the day of HRT-FET. Among women who did not conceive, there was no significant difference in the mean progesterone level on the day of the HRT-FET between those with endometriosis and those without (p = 0.709). Limitations, reasons for caution The primary limitation of our study is associated with its observational design. Extrapolating our results to other laboratories or different routes and/or dosages of administering progesterone also requires validation. Wider implications of the findings This study shows that patients diagnosed with endometriosis do not require higher progesterone levels on the day of a frozen blastocyst transfer to achieve a LB in hormonal replacement therapy cycles. Trial registration number NA

P-649 Unconventional action of Luteinizing Hormone

Abstract Study question Has Luteinizing Hormone (LH) a role in the activation/survival of follicles of the primary ovarian reserve? Summary answer LH promotes survival and perhaps activation/growth of primordial and primary follicles, proliferation of granulosa cells of secondary follicles, reduces follicle atresia at later growing stages. What is known already In the mammalian ovary, Primordial Follicles (PMFs) constitutes the primary ovarian reserve, which declines over time due to physiological events such as atresia, recruitment, and ovulation. While in the adult, the role of LH in steroidogenesis and ovulation is well established, recent work from our laboratory has shown that under chemotherapeutic treatments, LH supports PMFs survival in prepuberal mouse ovary. Study design, size, duration LH effects on follicle dynamics and some activities of pre-granulosa cells were investigated in vitro and in vivo. In vitroexperiments were performed on PD4 GFP/Kit mice ovarian fragments and on PD8 CD1 mice ovarian somatic cells cultured up to six days in continuous presence of 0.2, 0.5, 1IU LH. In vivo experiments were performed on 9 PD4 and 8 PD28 CD1 mice receiving an intra-peritoneal injection for eight consecutive days of 1-5IU LH. Participants/materials, setting, methods Number and diameters of the GFP stained oocyte were counted in the ovarian fragments. Kit-Ligand and LH-R mRNA expression levels were analyzed by qPCR in pre-granulosa cells. For in vivo experiments, ovarian volume was measured by Micro-Ct and follicle counts were performed on histological sections. Estrous cycle phases were monitored by vaginal smear during the 8 days of treatment. Statistics was performed using Student’s t-test. Main results and the role of chance In fragments of PD4 mouse ovaries cultured in vitro for six days, 1IU LH significantly increased the number of oocytes with a diameter >40 mm (CTRL 17.7% vs LH 36.07%, p < 0,05), suggesting activation of PMFs. Moreover, the hormone increased the expression of mRNA for LHR and KL in the somatic ovarian cells and promoted their proliferation in vitro. The effects of 1IU LH on PD4 females can be summarized as follows: 1. Ovary volume increase (CTRL 0.3 mm3 vs LH 0.36 mm3, p = 0.07), 2. higher percent of PMFs (CTRL 68.227% vs LH 78.29%, p < 0,05) associated with lower percent of primary (CTRL 7.2% vs LH 4.05%, p < 0.04) and growing follicles (CTRL 14.18% vs LH 8.56%, p = 0.1), 3. higher number of granulosa cell layers (> three layers) in secondary follicles (CTRL 42.8% vs LH 54.6%), and 4. lower percent of atretic follicles (CTRL 2.64% vs LH 1.72%, p = 0.1). Similar results were obtained on 28 dpp mice. Morever, we observed that corpora lutea were present only in ovaries of LH-treated females and that all such females were in estrus. Limitations, reasons for caution The study was conducted only in mice. Wider implications of the findings These studies have highlighted new possible roles of LH on follicle classes (primordial/primary) whose development is considered gonadotropin independent. The results, if confirmed in humans, imply that LH might be used as a treatment for patients with ovarian disorders involved the survival/dynamics of the ovarian reserve. Trial registration number NA

P-664 Insulin-like peptide 3 as a potential biomarker in predicting IVF success and ovarian reserve in women with unexplained infertility and diminishing ovarian reserve

Abstract Study question Could insulin-like peptide 3 (INSL3) predict ovarian reserve and in vitro fertilization (IVF) success in women with unexplained infertility (UI) and diminishing ovarian reserve (DOR)? Summary answer In women with DOR, reduced intrafollicular and serum INSL3 levels correspond with low AMH and high FSH levels, similar to established ovarian reserve markers. What is known already INSL3, belonging to the insulin-like peptide family, is produced by theca interna cells within antral follicles and the corpora lutea. It is hypothesized that INSL3 is integral to the initial development and function of antral follicles, specifically through its regulatory effect on androgen biosynthesis in the thecal cells of these follicles. Moreover, INSL3 is implicated in the modulation of the ovarian microenvironment, which is essential for facilitating the maturation of oocytes. Study design, size, duration This research was carried out at the IVF centers of two tertiary university hospitals, assessing 49 infertile women with either UI (n = 24) or DOR (n = 25) during IVF treatment. A control group (n = 26) comprised women with normal ovarian reserve undergoing IVF due to male or tubal factors. Outcome parameters are ovarian reserve markers (serum FSH, estradiol, AMH, INSL3 concentration and antral follicle count [AFC]), positive pregnancy rate and live birth rate (LBR) (gestational age ≥28 weeks). Participants/materials, setting, methods Women aged between 20-40 with a body mass index (BMI)<30 kg/m² were included. Serum FSH, estradiol, AMH, INSL3 levels and AFC were evaluated on menstrual cycle days 2 or 3, and INSL3 in follicular fluid from dominant follicles (>14 mm) were evaluated during oocyte retrieval. Exclusions included endocrine abnormalities (e.g., polycystic ovary syndrome, hyperprolactinemia, hypo and hyperthyroidism) cycle cancellation due to lack of a viable embryo, uncorrected Mullerian anomalies, recurrent miscarriage history and hydrosalpinx presence. Main results and the role of chance The mean age in the study groups was 28.41±3.5 (UI), 32.40±4.21 (DOR), and 29.65±3.91 (control). In the DOR group, female age (p = 0.001), BMI (p = 0.049), duration of infertility (p = 0.006), previous IVF attempts (p < 0.001), basal FSH (p < 0.001), and basal estradiol (p < 0.001) were higher, while AMH (p < 0.001), AFC (p < 0.001), and follicular fluid INSL3 (p < 0.001) were lower. Serum INSL3 in DOR was lower but not statistically significant. No correlation was found between serum INLS, follicular fluid INSL3, ovarian reserve markers, positive pregnancy rates, miscarriage, and LBR. DOR group had longer stimulation durations (p = 0.017), but lower max estradiol (p < 0.001), fewer oocytes (p < 0.001), mature follicles (p < 0.001), and PN (p < 0.001). Pregnancy rates were 54.1% (UI), 24% (DOR), and 53.84% (control); LBRs were 54.1% (UI), 20% (DOR), and 42.3% (control), with significant differences in pregnancy tests (p = 0.04) and LBR (p = 0.03), both lower in DOR. Univariate analysis identified duration of infertility (p = 0.019) and basal FSH (p = 0.009) as significant LBR predictors. Multivariable analysis confirmed basal FSH (p = 0.033) as significant, with increased duration of infertility and basal FSH level reducing LBR probability. Limitations, reasons for caution The limited sample size of our study potentially undermines the robustness of circulating INSL3 as a predictive biomarker for DOR, evidenced by the non-significant decrease in serum INSL3 levels in the DOR group. Moreover, the lack of INSL3 level assessment during oocyte retrieval further constrains our findings. Wider implications of the findings Our research explored INSL3 as a viable biomarker for predicting ovarian reserve and IVF success in UI and DOR cases. Reduced INSL3 levels in circulation and follicular fluid in DOR patients could be beneficial in clinical practice. Trial registration number not applicable

P-708 melatonin alleviates circadian rhythm disruption-induced enhanced luteinizing hormone pulsatility and ovarian dysfunction in vivo and in vitro

Abstract Study question Is melatonin deficiency a reversible cause of abnormal pattern of luteinizing hormone (LH) pulsatility and ovarian dysfunction caused by circadian rhythm disruption (CRD)? Summary answer Melatonin could rescue the augmented LH pulsatility and reproductive abnormalities caused by CRD. What is known already Melatonin, mainly synthesized and released by pineal gland, engages in regulating circadian rhythms. When circadian rhythms are disrupted, the nocturnal productions of melatonin are suppressed. Evidence suggests that circadian rhythm disruption (CRD) is associated with reproductive dysfunctions, while melatonin plays a vital role for female reproduction as a free radical scavenger and a neuroendocrine regulator. The gonadotrophin-releasing hormone (GnRH) drives pulsatile luteinizing hormone (LH) secretion, which is also essential for fertility. Study design, size, duration We used 90 CBA/CaJ mice to establish the chronic jet-lag model in which mice were exposed to an 8 hr time shift every 3 days. After 8 weeks of jet-lag induction, some of the mice were given melatonin treatment for 4 weeks. Furthermore, we used a perfusion system in vitro to mimic the process of CRD-induce high-frequency GnRH pulses stimulation of LβT2 gonadotrope cells. Participants/materials, setting, methods Repetitive tail-tip blood collection was used to evaluate the pattern of LH pulsatility. Blood plasma samples were collected from 4 mice at 4-hour intervals to measure levels of melatonin. Follicular development was evaluated by ovarian histology. Reproductive hormones was detected by ELISA. Immunohistochemistry and western blot were used to detect the expression of melatonin receptor 1A (MT1). The LβT2 cells were exposed to pulsatile GnRH at high pulse frequencies to mimic augmented pulsatile GnRH stimulation. Main results and the role of chance CRD mice exhibited reproductive-endocrine dysfunction, including irregular estrous cycles, reduced number of corpus lutea (CL), and decreased levels of estradiol and progesterone. Additionally, CRD enhanced the frequency and amplitude of LH pulsatility and pituitary LH beta-subunit expression. Melatonin administration alleviated these CRD-induced reproductive abnormalities and reversed the augmented LH pulsatility. The expression of MT1 decreased in the anterior pituitaries of CRD mice but increased after melatonin treatment, indicating the essential role of MT1 in melatonin’s reversal of CRD-induced enhanced LH pulsatility. Limitations, reasons for caution Further research is needed to validate the applicability and safety of these findings in humans. Wider implications of the findings The results suggest that melatonin may be a beneficial option for patients with reproductive-endocrine abnormalities, particularly those with circadian rhythm disruption. Trial registration number 82101719

P-678 Cabergoline has inhibitory effects on FSH-induced cell differentiation and estrogen secretion in a primary culture system of rat ovarian granulosa cells

Abstract Study question How does cabergoline have effects on cell differentiation and hormone secretion induced by FSH stimulation in ovarian granulosa cells? Summary answer Cabergoline suppresses LH receptor expression and Estradiol (E2) secretion in primary cultures of rat ovarian granulosa cells by inhibiting cAMP production induced by FSH stimulation. What is known already Cabergoline is a dopamine receptor 2 agonist that has been used to treat hyperprolactinemia and, more recently, to prevent ovarian hyperstimulation syndrome. Although it has been previously reported that cabergoline suppresses E2 and Progesterone secretion induced by FSH stimulation in a primary culture system of rat ovarian granulosa cells, but the effect on LH receptor expression has not been investigated. It has also been reported that the activation of dopamine receptor 2 suppresses VEGF secretion and reduces the incidence of early-onset OHSS by inhibiting the VEGF/VEGFR2 pathway in research using human luteinized granulosa cells. Study design, size, duration We used a primary culture system of ovarian granulosa cells obtained from immature female rats treated with diethylstilbestrol to examine the effect of cabergoline addition on the changes in ovarian granulosa cells induced by FSH stimulation. Participants/materials, setting, methods FSH and cabergoline were added alone or co-added to a primary culture system of rat ovarian granulosa cells, and the changes induced by them were measured and compared. (1) mRNA expression of Lhcgr, Cyp19a1, and Fshr by RT-qPCR (2) E2 concentration in the culture medium (3) Measurement of hCG binding on the cell surface (4) cAMP concentration Main results and the role of chance Co-addition of cabergoline suppressed each of the changes induced by FSH stimulation as follows. (1) The mRNA expression levels of Lhcgr, Cyp19a1, and Fshr decreased in a concentration-dependent manner (0.01-10 µM) of co-added cabergoline compared to FSH alone. In the 10 µM of co-added cabergoline group, they decreased to 14%, 9%, and 49% of FSH alone, respectively. (2) E2 concentration in the culture media also decreased in a concentration-dependent manner (1-10 µM) of co-added cabergoline compared to FSH alone. In the 10 µM of co-added cabergoline group, it decreased to 49% of FSH alone. (3) The amount of hCG binding on the cell surface also decreased in a concentration-dependent manner (0.1-10 µM) of co-added cabergoline compared to FSH alone. In the 10 µM of co-added cabergoline group, it decreased to 7% of FSH alone. (4) FSH-stimulated cAMP production decreased in a concentration-dependent manner (0.01-1µM) of co-added cabergoline compared to FSH alone. In the 1µM of co-added cabergoline group, it decreased to 50% of FSH alone. These results suggest that cabergoline suppresses LH receptor and aromatase expression and E2 production by inhibiting cAMP production induced by FSH stimulation. Limitations, reasons for caution Since this is an in vitro experiment using a rat primary culture system, the results of this research cannot be considered directly applicable to humans. Wider implications of the findings Our findings imply that cabergoline administration during the follicular phase may inhibit ovarian granulosa cell differentiation and estrogen secretion. In women taking relatively high doses of cabergoline, attention may need to be paid regarding the effects of cabergoline on follicular development and estrogen secretion. Trial registration number not applicable

O-310 PARP inhibitors have no toxic effect on follicle reserve, mitotic and steroidogenic function of granulosa cells in human ovary

Abstract Study question Do the PARP inhibitors have gonadotoxic effects on human ovary? Summary answer No. The PARP inhibitor drugs Olaparib and Niraparib do not appear to have any toxic effects on the human ovary. What is known already The Poly (ADP-ribose) polymerase (PARP) family has many essential functions in cellular processes, including the regulation of transcription, apoptosis, and the DNA damage response. PARP has Poly (ADP-ribose) activity, which facilitates the recruitment of other repair proteins to promote the repair of DNA single-strand breaks when DNA damage occurs. PARP inhibitors are approved for the treatment of BRCA1/2 mutated breast and ovarian cancers. No data is available regarding the gonadotoxic potentials, if any, of these drugs on the human ovary. We aimed to address this issue in the current study. Study design, size, duration A clinical translational research study that utilizes ex-vivo and in-vitro experimental models on human ovarian cortical samples, primary luteinized non-mitotic granulosa cells, and immortalized human granulosa cells. Participants/materials, setting, methods Ovarian cortical samples were obtained from young women undergoing laparoscopic surgery for benign ovarian cysts (n = 5). Human non-mitotic luteinized granulosa cells (HLGCs) obtained from 10 IVF patients during the oocyte retrieval procedure and immortalized mitotic non-luteinized granulosa cell line (HGrC1), and immortalized granulosa cell tumor cell line (COV434) were used for the experiments. Cell/tissue culture, confocal imaging, flow cytometry, immunoblotting, cell viability/proliferation and hormone assays were applied. Main results and the role of chance Treatment of ovarian tissue samples and granulosa cells with PARP inhibitors at five different therapeutic concentrations (1, 5, 10, 20, and 40 µM/mL) up to 96 hours in culture resulted in resulted in the cleavage of PARP, validating in vitro activity of the drugs. The primordial follicle reserve and steroidogenic activity of the tissue samples remained stable at the end of culture period. Similar results were obtained in the granulosa cells, which retained their viability and steroidogenic functions after exposure to the PARP inhibitors. Real-time impedance-based cell proliferation assay with xCelligence system revealed that granulosa cells continued to proliferate normally and reached log phase in the presence of PARP inhibitors without any discernible cytotoxic effect. In immunoblot analysis, the expression of steroidogenic enzymes (StAR, aromatase, 3B-HSD and 17B-HSD) was comparable to control cells and cleaved caspase-3 expression was absent in the granulosa cells exposed to PARP inhibitors. Limitations, reasons for caution Caution should be exercised before extrapolating our data because these findings were obtained in ex-vivo and in-vitro models. They need to be confirmed using in-vivo experimental models such as human ovarian tissue xenografting in nude mice. Wider implications of the findings This study provides the first reassuring molecular evidence that PARP inhibitor drugs do not appear to have a significant gonadotoxic effect on human ovary. Trial registration number not applicable

P-585 Vaginal progesterone associated with oral dydrogesterone versus vaginal progesterone for luteal support in letrozole-hcg cycles for thawed embryo transfer: a non-inferiority prospective cohort study

Abstract Study question Does micronized vaginal progesterone (MVP) associated with oral dydrogesterone (DYG) an alternative for luteal support in Letrozole-hcg cycles for thawed embryo transfer? Summary answer The association of MVP and DYG is a non-inferior alternative to only MVP for luteal support in thawed embryo transfer in Letrozole-hcg cycles. What is known already Endometrial preparation is essential for thawed embryo transfer. Several protocols involving different luteal support with progesterones are available. Moreover, the presence of corpus luteum decreases the obstetrical morbidity mainly linked to hypertensive disorders. Letrozole-hcg administration is a good alternative for frozen embryo transfer (FET). However, the literature lacks information about the comparison among the different protocols for luteal support in Letrozole-hcg cycles. Dydrogesterone is an oral progesterone with some unique characteristics which could be utilized with some advantage for luteal support, including oral administration and some immunological activity. Study design, size, duration This is a prospective non-inferiority cohort study performed at an infertility clinic including patients performing the first FET cycle during 2023. Reproductive outcomes were compared between patients receiving 600 mg (3x200 mg) of MPV or 40 mg of DYG plus 400 mg of MVP after Letrozole-hcg administration. Participants/materials, setting, methods All patients (aged 23-39 years) undergoing their first FET cycle during 2023 were included in this study (n = 169), divided into two groups: MVP+DYG (n = 54) or MVP (n = 115) for luteal support after Letrozole-hcg. Letrozole was administered for 5 days (2,5 mg/bid) and hcg was done when a dominant follicle reached 18 mm with a follicular endometrium measuring at least 7 mm. Main results and the role of chance Both groups were equivalent in terms of age, infertility aetiology, BMI and AMH (P > 0.05). Moreover, the proportion of cleavage or blastocyst stage was also similar between both groups (57% blastocyst transfer for MVP+DYG and 50% blastocyst transfer for MVP group, P > 0.05). The number of transferred embryos was (mean ±SD) 1.6±0.5 for both groups, P > 0.05. Furthermore, the number of top-quality embryos (cleavage stage or blastocyst) was also similar, P > 0.05. Moreover, clinical pregnancy (> 12 weeks) was 42% in MVP+DYG group versus 31% in the MVP group (-0.22-1.27, P = 0.156) and the early pregnancy loss per positive hcg test did not differ (18% for MVP+DGY and 22% for MVP, P = 0.687). In addition, we performed a multivariable analysis controlling possible confounders for clinical pregnancy: BMI, age, AMH, embryo quality, infertility aetiology and embryo stage during the transfer, and only age (0.86; 95%CI -0.229 to -0.064, P = 0.001) was related to clinical pregnancy. Limitations, reasons for caution The main limitation is the cohort design of our study, with an inherent risk of bias (selection bias). Moreover, we calculated this study to achieve the power for a non-inferiority comparison, not superiority comparison. Wider implications of the findings This is the first study comparing MVP+DYG to only MVP in patients submitted to FET using the Letrozole-hcg protocol. We demonstrated in this non-inferiority study the usefulness of adding DYG. Moreover, our results were promising showing a trend towards better pregnancy rates after MVP+DYG. Trial registration number not applicable

P-626 Progesterone rescue is effective for clinical pregnancy outcome in women presenting low circulating progesterone levels at frozen embryo transfer day

Abstract Study question Is subcutaneous progesterone (prolutex® 25mg daily) addition effective for clinical pregnancy outcome in women presenting low progesterone(P4) levels at frozen embryo transfer(FET) day? Summary answer Subcutaneous progesterone rescue improved ongoing pregnancy rate and clinical pregnancy rate in women with low P4 results at FET day. What is known already FET has been increased, because preimplantation genetic testing for aneuploidies is performed widely, and FET presents a significantly low risk of ovarian hyperstimulation syndrome.In FET, there is no established optimal protocol for preparing endometrium, but hormonal replacement therapy (HRT) is widely used due to its convenient schedule adjustment. HRT protocol induce anovulation, functioning corpus luteum is lacking and exogenous progesterone administration is very important. Therefore, women showing low P4 levels around FET day have lower pregnancy rate. However, there are currently no data on the effectiveness of progesterone rescue. Study design, size, duration Primary outcomes from a randomized cohort retrospective study investigating subcutaneous progesterone (prolutex® 25mg) daily addition for whom had low P4 levels (<12ng/mL) at FET day. The study was conducted at one infertility center in Korea from January 2022 to December 2023 and pregnancy outcomes of FET transfers registered until Oct. 2023,. 194 patients fulfilled the inclusion criteria. Participants/materials, setting, methods One hundred ninety four women divided into two groups based on progesterone levels, with 12ng/ml as the threshold. Under 12ng/mL group were randomized to subgroup with and without addition of subcutaneous progesterone. In total 278 FET cycles, the effect of progesterone rescue on cumulative clinical pregnancy rate (CPR), Ongoing pregnancy rate (OPR) was reported as primary outcomes. Main results and the role of chance In total 278 cycles, there were 137 cycles with adequate serum P4 levels, while 141 cycles presented with low P4 levels in which randomly rescue subcutaneous progesterone administration added from next day of embryo transfer. When comparing the groups, there was no statistical difference in maternal age, BMI, smokers, duration of infertility, type of infertility, indication of in vitro fertilization(IVF), endometrial preparation method. In low P4 group, with and without subcutaneous progesterone addition subgroups included 51 and 90 cycles, retrospectively. CPR were 52.9% versus 35.6% (P = 0.044; chi-squared test) with a statistically significant higher rate of OPR in progesterone rescue group (45.1% versus 28.9%, p = 0.052). When comparing the group with adequate P4 levels and the group with low P4 levels without progesterone rescue, OPR were 18.2% versus 28.9% (p = 0.06, chi-squared test), CPR was significantly higher in low P4 levels group (23.4% versus 35.6%,p=0.046, chi-squared test). Adequate P4 level group had more IVF attempts (4.6 versus 2.2, p < 0.001, chi-squared test). There is tendency to use more luteal phase support medication with repeated attempt (serum P4 concentration on ET day 39.3 ng/mL versus 8.7 ng/mL, p < 0.001, t-test) Limitations, reasons for caution The main limitation of the study was retrospective, and had small sample size. A control group without progesterone rescue in low P4 levels did not receive any intervention. Wider implications of the findings The progesterone rescue for women presenting low P4 on FET day improved clinical outcome and excessively raising progesterone medication may have adverse effects. Therefore rather than using excessive progesterone administration from luteal support start day, progesterone rescue might be effective when low P4 levels presenting at FET day. Trial registration number not applicable

P-426 Comprehensive study of the endometrial early/mid secretory phase during natural cycles reveals conserved molecular dynamics involved in the window of implantation

Abstract Study question Is there a common transcriptomic signature in the endometrial early/mid secretory phase of healthy women in natural cycles? Summary answer We observed a conserved transcriptomic regulation of the endometrium around the window of implantation (WOI), consistent across donors, and not influenced by the biopsy procedure. What is known already Extensive research has been conducted on the gene expression changes that characterize the WOI, aiming to identify signals that indicate its onset. However, much of this work has utilized whole-transcriptome techniques and has involved IVF patients, who require luteal phase support. The few studies performed in natural cycles often relied on restricted methods, such as qPCR that examines only a few targets simultaneously. As such, an extensive transcriptomics study in volunteer donors in true natural cycles is lacking. This may provide deeper insights into the physiological changes during the WOI in young women driven by the endogenous effects of progesterone. Study design, size, duration 34 healthy donors were enrolled. Each donor provided one endometrial sample, on a day between LH + 2 and LH + 9 during their natural cycle. A minimum of two and a maximum of five donors were allocated to each day. Ovulation was detected via ecographic inspection and hormonal determination of LH/E2/P4. LH + 0 was established when the LH peak was detected (>2.8 times basal levels), together with a subsequenc decrease in E2 (-30%) and increase in P4 (>1.6ng/ml) Participants/materials, setting, methods When the biopsy was of sufficient size, we separated it in two parts and extracted RNA separately to assess concordance between different areas of the same biopsy. RNAseq libraries were prepared with NEBNextUltraII Directional RNA LibraryPrep and sequenced on a NovaSeq6000. Fastq files were aligned with STAR to GRCh38. Principal component analyses (PCA) were created with plotPCA, gene expression time course analysis was performed with DEseq2, and pseudotime analysis was performed using the slingshot package. Main results and the role of chance Of the 34 donors, we obtained good quality RNAseq data (minimum 20 million reads) from 41 samples, with 27 donors having one sample sequenced, while 2 independent samples from the same biopsy were obtained from 7 volunteers. Analysing same-biopsy concordance via PCA revealed a great degree of similarity between all the pairs, which clustered close to each other. Samples from different donors collected on the same day also showed a good clustering, apart from a single day LH + 4 sample. Pseudotime analysis revealed a distinct trajectory in transcriptomic profiles changing progressively from day LH + 2 to LH + 9. This analysis clearly delineated three phases: LH + 2 to LH + 5, LH + 6 to LH + 7, and LH + 8 to LH + 9. Accordingly, we categorized these into pre-receptive, receptive and post receptive groups, and conducted a time course experiment. We identified a total of 3,311 genes with varying expression across these three timepoints, primarily associated with progesterone signalling, metabolism and cell-to-cell communication. Based on their expression patterns, we isolated a subset of 538 genes which increased their expression during the receptive phase only. These genes could represent novel markers for a more in-depth characterization of the WOI. Limitations, reasons for caution The samples analysed are in bulk, and the genes identified could be expressed either in the stroma or in the epithelial compartment. These findings are not applicable to artificial cycles, as the absence of the corpus luteum and varying levels of exogenous progesterone administered may change the transcriptomic signatures. Wider implications of the findings Our study revealed good concordance of whole transcriptome profiles of donor’s endometria according to cycle progression. This may lead to the identification of novel markers involved in the regulation of the WOI that may not have been found in previous studies performed on artificial cycles. Trial registration number not applicable

P-693 Three-dimensional modeling of follicular microenvironment dynamics in the human adult ovary

Abstract Study question How does the ovarian microenvironment, involving neural fibers, vascular networks, and immune cells, interplay with follicular dynamics in the human adult ovary? Summary answer Neural fibers growing into the ovarian cortex and vascular network expansion were associated with primordial follicle activation and growth. Macrophages associated with antral follicular atresia. What is known already The ovarian follicles, responsible for preserving and supplying oocytes, play a crucial role in fertility, with their fate influenced by the ovarian microenvironment. However, the three-dimensional (3D), irregular, and dynamic structural changes of the ovarian microenvironment are difficult to interpret using two-dimensional analysis. 3D-imaging technologies have been developed to reveal intricate structural and functional relationships particularly in rodents, but these approaches have not been widely adopted for human ovarian research. Study design, size, duration Our study aims to explore the follicular microenvironment dynamics in the human adult ovary. In this work, we optimized whole-mount immunofluorescence using TUBB3, PODXL, PECAM1, ACTA2 and CD68, and conducted 3D imaging on intact adult ovarian tissue, offering a visualization of the 3D structures and dynamic changes within ovarian follicles, neural fibers, vascular networks, and immune cells in human adult ovary at the single-cell level. Participants/materials, setting, methods The human ovarian samples were collected from donors (N = 5) who underwent oophorectomy after testosterone treatment. Part of the intact ovary, including cortex and medulla, was fixed and used for whole-mount immunofluorescence using markers for neural fibers (TUBB3), blood vessels (PODXL, PECAM1, ACTA2), and immune cells (CD68). Subsequently, an enhanced iDiSCO method was employed for tissue clearance. Finally, 3D images were acquired using a lightsheet microscope, followed by image analysis and 3D reconstruction using Imaris software. Main results and the role of chance Our results showed that TUBB3-labeled nerve fibers predominantly distributed in the ovarian cortex, with few presented in the medullary region. Notably, their presence surrounding secondary and antral follicles was limited. We observed a rich distribution of PODXL-labeled capillaries surrounding primordial and primary follicles in the cortical layer of the adult ovary. As follicles progressed to the secondary stage, PECAM1-labeled vasculature contributed to the construction of a vascular network enveloping the secondary follicles and connecting extensively to larger-caliber arteries containing (ACTA2+) smooth muscle cells. In antral follicles, large larger-caliber arteries were also detected, maintaining proximity to the theca cell layer. Interestingly, in cases of follicular atresia, vascular invasion within the follicular cavity was observed. Additionally, clusters of CD68+ macrophages exhibited a scattered distribution around the follicles at various stages, with a significant accumulation in atretic small-antral follicles. Limitations, reasons for caution Our study was conducted on ovaries from a limited number of donors after gender-affirming surgery. Although we aimed to provide insights into the ovarian microenvironment, we were so far unable to obtain ovaries with large antral follicles, ovulatory follicles and the formation of the corpus luteum. Wider implications of the findings The views provided by 3D-image analysis of the human ovary will augment the current knowledge of the complex interactions between the follicles and the ovarian microenvironment. 3D ovarian mapping will also provide a roadmap for generating oocytes in vitro for fertility preservation and ovarian disease modelling. Trial registration number N/A.

P-435 Superiority of modified natural cycle vs hormonal replacement therapy in non-menopausal egg recipients of advanced maternal age: clinical outcomes from 1273 single blastocyst transfer cycles

Abstract Study question Is the modified natural cycle (mNC) better than hormonal replacement therapy (HRT) for endometrial preparation in non-menopausal egg recipients of advanced maternal age? Summary answer In women above 40 years, mNC shows better reproductive and obstetrical outcomes compared to hormonal replacement therapy, especially in the subgroup 45-50 years old. What is known already There is extensive evidence showing the superiority of mNC over HRT in terms of live birth rates, miscarriages, and even obstetric complications. Apparently, the presence of the corpus luteum plays a major role in endometrial receptivity and embryo implantation, probably allowing for better placentation. Nevertheless, there is a general belief that the ovulatory cycle of older women is not the best choice for endometrial preparation, although paradoxically they are the group of patients who could benefit most from the reduction of obstetric complications. An increase in cancellation rates for mNC is claimed, without consistent data available. Study design, size, duration Retrospective single-centre comparative clinical study, including 1273 endometrial preparation cycles for frozen single-blastocyst transfer between January 2020 and December 2022: 892 (70,2%) on HRT protocol and 381 (29,8%) on mNC protocol. The two groups were homogeneous for donor’s age (25,7 years HRT vs 25,3 years mNC), recipient’s age (42,2 years HRT vs 42,1 years mNC), severe male factor (6% HRT vs 5% mNC), and the need for a sperm donor (28% HRT vs 26% mNC). Participants/materials, setting, methods Among 1273 started non-menopausal recipient’s cycles, we compared cancellation rates between HRT and mNC due to either medical reasons (inadequate endometrium, bleeding, premature ovulation) or other issues (no embryo survival, COVID-19, etc). We studied the outcomes of the 1096 elective frozen single-blastocyst transfers performed, comparing HRT and mNC cycles for: pregnancy, clinical pregnancy, miscarriage, live birth and <35weeks preterm birth rates. We stratified the results by age and used Chi-square test to compare groups (p < 0,05). Main results and the role of chance The overall cancellation rate was 13.9%. No significant differences in cancellation for medical reasons were found between HRT (7%) and mNC (6.8%), p = 0.93. When we analyse results stratified by age, patients aged 39 or less (270 cycles: HRT 75.9%, mNC 24.1%) showed no statistically significant differences in pregnancy (58% vs 64.6%, p = 0.35), clinical pregnancy (50.7% vs 56.9%, p = 0.34), miscarriage (18.3% vs 18.9%, p = 0.93), and live birth rates (41.5% vs 46.2%, p = 0.5). In the 40-44 age group (494 cycles: HRT 64.2%, mNC 35.8%), no statistically significant differences were found between HRT and mNC in pregnancy (58.2% vs 61.5%, p = 0.47), clinical pregnancy (50.8% vs 51.1%, p = 0.96), and live birth rates (40.4% vs 45.8%, p = 0.25). The HRT group had a significantly higher miscarriage rate (21%) compared to mNC (10%), p = 0.03. For patients aged 45-50 years (332 cycles: HRT 72%, mNC 28%), no statistically significant differences were observed in pregnancy (56.5% vs 62.4%, p = 0.33) and miscarriage rates (24.3% vs 15.1%, p = 0.18). Clinical pregnancy (44.8% Vs 57%, p = 0.04) and live birth rates (33.9% vs 48.4%, p = 0.01) were significantly higher in the mNC group compared to HRT. No significant differences were found between HRT and mNC in preterm birth rates <35 weeks (3.6% vs 4.6%, p = 0.75). Limitations, reasons for caution This study is a single-centre retrospective comparative trial. Our homogeneity study included several confounding variables, however other bias such as severe uterine factor, embryo quality or repeated implantation failure cases were not taken into account. Wider implications of the findings Modified natural cycle should be encouraged as the first option for endometrial preparation in non-menopausal egg recipients of advanced maternal age, having shown superiority over hormonal replacement therapy in patients aged 45-50 years. Moreover, modified natural cycles are more patient friendly and cost-effective, with no increase in the cancellation rates. Trial registration number not applicable

P-672 Is there a sweet spot for follicular size and/or estradiol level for the initiation of micronized progesterone to optimize natural proliferative phase (NPP) cryo-cycle outcome?

Abstract Study question Is there an optimal follicular size and/or estradiol level for the initiation of micronized progesterone to optimize outcomes in NPP cycles potentially restricting scheduling flexibility? Summary answer All follicular sizes >12 mm and estradiol levels >120 pg./mL do not significantly correlate with ongoing pregnancy rates (OPR) or pregnancy loss (PL). What is known already Planning of a frozen embryo transfer (FET) is feasible within either a natural (NC) or artificial cycle (AC). While AC-FETs offer flexibility in planning, concerns have raised due to studies indicating a higher risk for obstetric complications. Conversely, NC-FETs show a protective effect against these complications but are usually less flexible in planning and ask for more monitoring. Recently, the natural proliferative phase (NPP) protocol was described, in which luteal phase support is initiated as soon as the endometrium reaches 7mm. NPP-FETs were associated with ongoing pregnancy rates comparable to AC- and NC-FETs. Study design, size, duration A retrospective single-centre study including a total of 300 consecutive single frozen embryo transfers (FET), performed at a tertiary fertility centre between March 2023 and December 2023. All cycles were single blastocyst transfer cycles retained from IVF/ICSI cycles with autologous oocytes. Participants/materials, setting, methods A total of 227 patients underwent one or more frozen embryo transfer cycles. Administration of MVP 800mg BID was initiated after fulfilling the following criteria: follicular size > 12mm, estradiol level ≥120ng/mL and endometrium thickness ≥7mm. Blastocyst transfers were scheduled on the fifth day of MVP administration. Serum progesterone (P4) levels were determined on the day of transfer. Associations between treatment outcome, patient and cycle characteristics were explored using generalized estimating equations analyses. Main results and the role of chance The follicular size and estradiol level on the day of MVP initiation was not significantly associated with pregnancy rate (PR), ongoing pregnancy rate (OPR) or pregnancy loss (PL) (p = 0.386 and p = 0.260; p = 0.830 and p = 0.467; p = 0.978 and p = 0.313 respectively). Mean number of visits needed prior to the initiation of the MVP for scheduling the embryo transfer was 2.4 (SD 1.3). PR, but not OPR, was significantly associated with both the duration of the follicular phase as well as the number of visits (p = 0.008 and p = 0.006 respectively). For 91.3 % (274/300) of patients, embryo transfer could be scheduled on a weekday. 49% (146/300) of patients had a positive hCG test after the fresh blastocyst transfer in the NPP cycle. Ongoing pregnancy rate was 35 % (104/300), whereas pregnancy loss (early clinical miscarriage and biochemical pregnancy) was 23% (33/146). The OPR was significantly related to the age of the patient (p = 0.029). Both the PR as the OPR were significantly related the blastocyst quality (p = 0.008 and 0.009 respectively). The incidence of pregnancy loss was only significantly associated with blastocyst quality (p = 0.027). Mean serum P4 level at day of transfer was 19.4 ng/ml (SD 6.1) with a P4 level above 10ng/ml in 97% of cycles. Limitations, reasons for caution The retrospective character is a limitation of this study as well as the absence of live birth rate as a clinical outcome variable. Wider implications of the findings Flexibility about the required follicular sizes and estradiol levels open the perspective for exploring more flexible planning criteria such as lower thresholds for endometrial thickness. Further studies are needed to investigate whether ovulation occurs with formation of functional corpora lutea to effectively protect against hypertensive disorders. Trial registration number ONZ-2024-0015

HISTOLOGICAL STRUCTURE AND FUNCTIONAL PROPERTIES OF THE TUNICA ALBUGINEA OF THE OVARY

Just below the surface epithelium of the ovary is the tunica albuginea, which is a tight irregular connective tissue structure that gives the tissue its white color and contains fibroblast cells. Tunica albuginea, which is more resistant to environmental factors, contains fewer cells and is rich in collagen fibers, is observed as the niche of preantral follicles. It has been observed that fibroblasts forming collagen fibers provide the development of follicles with the paracrine factors and cytokines they secrete and function as a nourishing cell layer. After the graaf follicle forms the corpus luteum, fibroblasts in the adjacent tunica albuginea proliferate and the thickness of this structure increases. The tunica albuginea undergoes frequent renewal due to the corpus luteum structures formed in rats. When the corpus luteum is formed, new capillaries and venules are formed in the adjacent tunica albuginea. This structure may be a suitable model for investigating the migration of cells from the bone marrow to the ovary via vessels. It can be suggested that mesenchymal cells and very small embryonic/epiblast-like stem cells (VSELs), which show pluripotent stem cell characteristics, may migrate from the bone marrow to the tunica albuginea through vascular structures and that the bone marrow may be the source of these cells, which have been previously shown to be present in the ovary. The aim of this review is to examine the effects of the tunica albuginea on the development of follicles and the dynamic structure of the ovary.