Natural Cycle Research Articles

Low Dose hCG Support in Modified Natural Frozen Embryo Transfer Cycles.

What is the effect of a single low-dose recombinant hCG injection after embryo transfer (ET) in letrozole-induced modified natural frozen embryo transfer cycles (mNC-FET)?. An observational study was conducted in the university-affiliated referral clinic between 2022 and 2024. Women aged 18-42 with at least one vitrified blastocyst obtained from the previous cycle(s) were included. Ovulation induction for endometrial preparation was initiated with oral letrozol (5mg/day) for five days. Ovulation was triggered using 6500IU rec hCG sc when the leading follicle > 17mm, endometrial thickness > 7.5mm, and serum progesterone (P) < 1.5ng/ml. All women received 30mg dydrogesterone/day po for additional five-day luteal support. On the 6th day, ET was performed. Based on a quasi-randomized design, a group of women additionally received a half single bolus of (3250IU) rec hCG (sc) on the morning of 3rd day of ET (hCG group). Women who did not receive additional hCG were assigned as controls.One hundred fifty-four women were detected to be eligible for the study among 2150 initiated FET cycles during the period.Demographic data of the groups, including mean women's age, BMI, serum AMH, and infertility etiologies, were comparable in terms of variables. Mean serum progesterone values and the number of transferred embryos were also similar. A significantly higher ongoing pregnancy/started cycle was documented in the hCG group than in controls (46.7% vs 33.6% respectively, p = 0.03*). A single low-dose hCG injection after ET may improve the OPRs of women in letrozole mNC-FET cycles.

Electrocatalysts for Inorganic and Organic Waste Nitrogen Conversion.

Anthropogenic activities have disrupted the natural nitrogen cycle, increasing the level of nitrogen contaminants in water. Nitrogen contaminants are harmful to humans and the environment. This motivates research on advanced and decarbonized treatment technologies that are capable of removing or valorizing nitrogen waste found in water. In this context, the electrocatalytic conversion of inorganic- and organic-based nitrogen compounds has emerged as an important approach that is capable of upconverting waste nitrogen into valuable compounds. This approach differs from state-of-the-art wastewater treatment, which primarily converts inorganic nitrogen to dinitrogen, and organic nitrogen is sent to landfills. Here, we review recent efforts related to electrocatalytic conversion of inorganic- and organic-based nitrogen waste. Specifically, we detail the role that electrocatalyst design (alloys, defects, morphology, and faceting) plays in the promotion of high-activity and high-selectivity electrocatalysts. We also discuss the impact of wastewater constituents. Finally, we discuss the critical product analyses required to ensure that the reported performance is accurate.

O-206 Natural, modified natural or artificial endometrial preparation strategies for frozen embryo transfer in ovulatory women (MONART): a randomized controlled trial

Abstract Study question Are live birth rates after natural (NC), modified natural (mNC) endometrial preparation superior to artificial cycle (AC) strategies for ovulatory women undergoing frozen embryo transfer? Summary answer Live birth rates were comparable between NC, mNC, and AC. However, NC and mNC resulted in a significantly higher cycle cancellation rate. What is known already The number of frozen embryo transfer (FET) cycles has increased dramatically. In ovulatory women undergoing FET, three common strategies can be used for endometrial preparation: NC, mNC (NC + human chorionic gonadotropin trigger), and AC protocols. Previous studies comparing the effectiveness of these strategies had several drawbacks, including small sample size, lack of consistency between studies in the assessment and reporting of outcomes, and the retrospective origin, which introduced potential biases. Therefore, there is a lack of consensus on the optimal endometrial preparation protocol for FET. Study design, size, duration This single-center, open-label, randomized controlled trial was conducted at IVFMD, My Duc Hospital in Ho Chi Minh City, Vietnam. From March 2021 to March 2022, a total number of 1,428 participants were randomized (476 per group). The primary outcome of the study was live birth after one FET. Secondary outcomes were fertility outcomes, early pregnancy complications, obstetrical outcomes, perinatal and neonatal complications, and endometrial preparation cycle cancellation rates. Participants/materials, setting, methods Eligible participants were ovulatory women aged 18–45 with FET treatment, underwent no more than three IVF/intracytoplasmic sperm injection-FET cycles, had no more than two day-3 or one blastocyst transferred. Women with menopause/anovulation, contraindications for hormonal administration, uterine abnormalities who underwent in-vitro maturation, pre-implantation genetic testing, oocyte donation cycles were excluded. The FET strategies included FET after the first endometrial preparation cycles with AC, mNC, or NC, as per randomization, and the second with AC. Main results and the role of chance A total of 4,779 participants were screened, of whom 1,428 eligible individuals (476 per group) were randomized after written informed consent. The live birth rate after one FET was 36.6% (174/476) in the natural cycle strategy group, 33.4% (159/476) in the modified natural cycle strategy group, and 34.0% (162/476) in the artificial cycle strategy group (relative risk [95% confidence interval] for natural versus artificial cycle strategy: 1.07 [0.87–1.33] and for modified natural versus artificial cycle strategy: 0.98 [0.79–1.22]). Maternal and neonatal outcomes were comparable between groups, in particular,hypertensive disorders in pregnancy. For both natural and modified cycle groups, 20.08% (99/476) of the first FET cycles were canceled before subsequent successful ET versus none in the artificial cycle group. Limitations, reasons for caution Due to the nature of the intervention, blinding was not possible. The study was conducted in ovulatory women in a single center in Vietnam, which may limit the generalisability of the findings. Wider implications of the findings Live birth rates and obstetric/neonatal outcomes after the natural cycle, modified natural cycle, or artificial cycle strategies for ovulatory women undergoing FET are comparable, with higher cancellation rates in both the natural cycle and the modified natural cycle groups. Trial registration number NCT04804020

P-398 Lower ectopic pregnancy prevalence with natural endometrial preparation in single euploid frozen embryo transfers

Abstract Study question Does the endometrial preparation approach for an euploid-frozen embryo transfer have an impact on the incidence of an ectopic pregnancy? Summary answer The natural cycle endometrial preparation approach seems to have a lower incidence of ectopic pregnancies compared to a hormonal replacement therapy cycle. What is known already Ectopic pregnancy (EP) is an uncommon complication of pregnancy and is associated with significant morbidity. IVF pregnancies have been implicated as a risk factor, but this finding is mostly confounded by multiple embryo transfer and complicated medical history of women undergoing IVF treatment (tubal disease, surgery, endometriosis, etc.). Recent studies have suggested that frozen embryo transfer may reduce the risk of EP, but it is not yet known whether embryo ploidy status and different endometrial preparation regimens affect the risk of EP. Study design, size, duration Retrospective cohort of 1517 pregnancies from 2607 single euploid frozen embryo transfers. Comparator estimates were estimated using multiple large cohort studies in the literature including 58,203,239 pregnancies from unselected cohorts and 220,167 pregnancies from IVF-treated women with the transfer of non-tested embryos. Participants/materials, setting, methods The rate of ectopic per pregnancies were compared to ectopic rate derived from multiple cohorts of IVF-treated women and population level reports of unselected pregnancies (natural conception and IVF). Comparator estimates were obtained with proportion meta-analyses using generalized linear mixed models. Factors associated with ectopic pregnancies were investigated with mixed-effects logistic regression analyses. Main results and the role of chance EP rate in our cohort was 10/1000 pregnancies (95% CI: 5 to 16 per 1000) resulting from single euploid embryo transfers (1.1%, 16/1517). The observed prevalence of EP was 12/1000 pregnancies (95% CI: 6.8 to 21 per 1000) in unselected cohorts and 14/1000 pregnancies (95% CI: 13 to 15 per 1000) after IVF-treatment. Ectopic rate in our study was not significantly different than rates reported in the literature from unselected and IVF pregnancies (P = 0.579 and 0.223, respectively). There were no significant differences between women with ectopic pregnancies and those with clinical pregnancies in terms of age (33.6±5.5 vs. 35.1±5.2 years, P = 0.290), BMI (26.7±4.8 vs. 25.8±4.6 kg/m2, P = 0.475) and serum AMH levels (3.3±2.9 vs. 3.4±3.9 ng/mL, P = 0.854), nor between the quality of embryo transferred between top, good, fair and poor (1.8% vs 0.7% vs 1.9% vs 0.9%, p = 0.088, 0.997 and 0.508 respectively). Ectopic rates were also similar between women with history of cesarean section (1.1% vs 1.0%, P = 0.847) or isthmocele (1.0 vs 1.1%, P = 0.857). There were less ectopic pregnancies following transfers with natural endometrial preparation (0.5 vs 1.5%, natural vs programmed) and the effect showed statistical significance in multivariable analyses (OR: 0.28, 95% CI: 0.06 – 0.88, P = 0.049). Limitations, reasons for caution Small sample size as well as infrequent occurrence of the studied event reduces the power of our study to detect small but clinically meaningful effects with statistical significance. Wider implications of the findings We did not observe a lower than reported prevalence of EP among euploid embryo transfer. However, rate of EP was significantly lower in natural endometrial preparation cycles compared to preparation with exogenous hormones, indicating that a natural cycle might have a protective effect against the development of an ectopic pregnancy. Trial registration number Not applicable

P-388 The science of frozen embryo transfer, is modified natural cycle better?

Abstract Study question Do live birth rate, obstetric outcomes and number of clinic visits differ between frozen embryo transfer in modified natural and artificial cycle? Summary answer Modified natural cycle yielded a higher live birth rate for the similar number of visits, less hypertension, but higher rate of gestational diabetes. What is known already A frozen embryo transfer in a modified natural cycle entails procedure timing such that it coincides with the woman’s ovulatory cycle. Conversely, a frozen embryo transfer in an artificial cycle involves controlled hormonal stimulation, with the subsequent scheduling of embryo transfer. Based on the current evidence, there is no difference in pregnancy rates between modified natural cycle and artificial cycle. Frozen embryo transfer during an artificial cycle allows for greater flexibility for the patient and physicians. However, it seems to be associated with higher risk of adverse obstetric and neonatal outcomes. Study design, size, duration 1206 frozen embryo transfer cycles between January 1st, 2022, and December 31st, 2022 were retrospectively studied. Patients older than 40, with recurrent implantation failure, and recurrent pregnancy loss were excluded. Patients were divided according to their age, BMI, AMH, and the type of embryo transfer protocol. Compared outcomes were endometrial thickness, biochemical pregnancy rate, clinical pregnancy rate, miscarriage rate, number of clinic visits prior to transfer, obstetric complications, livebirth rate, and livebirth weight. Participants/materials, setting, methods Patients in the modified natural cycle group were followed by ultrasound and when the endometrium reached a thickness ≥ 7mm, and a dominant follicle ≥ 15 mm, HCG trigger was given, and the embryo (blastocyst) transferred 7 days later. In the artificial cycle group, patients received estrogen supplementation either orally or by patches and when the endometrium reached a thickness ≥ 7 mm, embryo transfer was scheduled following intramuscular progesterone administration for 6 days. Main results and the role of chance Patients with modified natural cycle (n = 250) embryo transfers have better clinical pregnancy rate compared to the embryo transfers (n = 250) in artificial cycles (63,6% vs 55,2% p = 0,05), and significantly better live birth rate (57,2% vs 43,9% p = 0,003) despite similar biochemical pregnancy rate (64% vs 61% p = 0,48). The miscarriage rate was significantly lower in modified natural compared to artificial cycles (10,2 % vs 20,99%, p = 0,008). Similar results were found with the transfer of PGTA tested embryos. There was no difference in the endometrial thickness between the 2 groups. The number of visits was higher in the modified natural cycle group, but the difference was not clinically significant (1,57 days vs 1,2 days). When comparing the obstetric outcomes, patients with a modified natural cycle transfer had a lower risk of hypertension (6,6% vs 13,5 % p = 0,10), but a significantly higher risk of gestational diabetes (19% vs 8,3% p = 0,02). No difference was observed regarding the birth weight, percentage of preterm birth or mode of delivery. Regression analysis was performed to identify any cofounder that might have affected the clinical pregnancy rate, and the results showed that the type of transfer was the only factor affecting the outcomes (OR: 1,23 CI: 1,03- 1,46) Limitations, reasons for caution The main limitation of this study is its retrospective nature. A randomized controlled study should be performed to confirm these findings. Wider implications of the findings Modified natural embryo transfer could be used to improve pregnancy outcomes. With proper scheduling, embryo transfer could be done with a similar number of visits for modified natural cycles and artificial cycles. More studies are needed to assess the increased risk of gestational diabetes with natural cycle embryo transfer. Trial registration number NCT06053827

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 (&amp;gt;2.8 times basal levels), together with a subsequenc decrease in E2 (-30%) and increase in P4 (&amp;gt;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

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 &amp;gt;180pg/ml, leading follicle &amp;gt;16mm and endometrial thickness was &amp;gt;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 (&amp;lt; 25th percentile [&amp;lt;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 (&amp;lt; 25thperc [&amp;lt;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-286 Comparative study of frozen-thawed embryo transfer with modified natural cycle versus HRT cycle in previously cancelled true natural cycle patients

Abstract Study question Is there any difference in outcome of frozen embryo transfer(FET) between hormone replacement therapy(HRT) and modified natural cycle(MNC) in previously cancelled true natural cycle patients Summary answer In patients with previous history of cancelled true natural cycle, MNC results in higher clinical pregnancy rates after FET, when compared to HRT cycle What is known already Preparation of endometrium lays down the foundation for conception and subsequent successful pregnancy.There has been a plethora of endometrial preparation protocols described in the literature. The established protocols are true natural cycle,MNC with or without luteal phase support(LPS) and HRT cycle with or without gonadotrophin releasing hormone agonist(GnRha) suppression. Nevertheless,there is no consensus on protocol which may provide the best outcome in patients who have had cancelled true natural cycle.In such situations,HRT cycle may be considered as an easy option as the cancellation rate is low.However,HRT treatment is known for higher early pregnancy and obstetric complications,compared to MNC Study design, size, duration Prospective observational study,where 155 patients undergoing FET who had minimum one true natural cycle cancellation were recruited.They were divided into group A undergoing HRT cycle(n = 77) and group B for MNC cycle(n = 78).The study was conducted in a fertility unit over six months from June-November 2023.Primary outcome measures were serum betaHCG estimation and ultrasonographic documentation of intrauterine pregnancy.Results were analysed using statistical software SPSS version25 and p value of less than 0.05 was considered statistically significant Participants/materials, setting, methods HRT patients received injectable GnRha 3.75 mg on day21 of previous cycle.Oral estradiol valerate 6mg daily started on day2 of menses. When ET was minimum 7mm, luteal phase support(LPS) was started with daily oral and vaginal progesterone(30mg and 600mcg respectively),followed by FET. MNC patients received Letrozole 5mg daily from day 2-6 of menstruation.rhCG 250mcg was given subcutaneously,when ET was at least 7mm,dominant follicle 16-18mm and progesterone(P4) less than 1ng/ml.LPS(vaginal progesterone600mcg) was started 36hours after rHCG,followed by FET Main results and the role of chance Mean ET in MNC group was 8.92mm ± SD1.38, which was higher than HRT group (8.31 mm ± SD0.89). This difference was statistically significant with a p value of 0.014.Serum beta HCG estimation was done 2weeks after FET. In HRT group beta HCG was positive in 41 out of 77(53.25%) and in MNC group positivity was higher being 48 out of 78(61.54%). However, this difference was not statistically significant. Transvaginal ultrasonography(TVS) was done to confirm intrauterine pregnancy 2weeks after beta HCG assay. Clinical pregnancy rate(CPR) was higher in MNC group with 32 out of 78(41.03%),compared to 18 out of 77(23.38%) in HRT group. This difference is statistically significant with p value of 0.019 Limitations, reasons for caution Relatively small population in this study may be the reason for caution to interpret the statistical data. Study with higher numbers may bring about more robust conclusions Wider implications of the findings Patients with failed true natural cycles may be considered for modified natural cycle(MNC) stimulation for FET, without attempt of another true natural cycle stimulation Trial registration number NOT APPLICABLE

P-613 Natural cycles monitoring for frozen embryo transfers: is it beneficial to start immediately after ovarian stimulation?

Abstract Study question Are there more advantages in performing frozen embryo transfer (FET) immediately after ovarian stimulation or in delaying it by at least one menstruation? Summary answer In natural cycles, timing of frozen embryo transfer does not affect clinical pregnancy rates, but immediate transfer is associated with higher rates of cycle cancellation. What is known already In order to achieve a reduced time to pregnancy and avoid additional stress for the couple, some Centers initiate FETs immediately, i.e. on the first menstrual cycle after ovarian stimulation. However, the detrimental effect of hormonal stimulation on ovarian function and endometrial receptivity may persist in the following cycle and immediate FETs may increase the possibility of cycle cancellation and reduce pregnancy rate. Available evidence on this subject, mainly focused on endometrial preparation with hormonal replacement therapy (HRT), showed contrasting results. Moreover, studies on FETs performed on natural cycles are lacking. Study design, size, duration This is a retrospective study conducted at the Infertility Unit of the Fondazione IRCCS Ca’ Granda Ospedale Maggiore Policlinico in Milan, Italy. A large amount of single FETs, performed between 2014 and 2021, was considered and was then divided in two groups according to their timing (immediate or delayed). The primary outcome was to assess the clinical pregnancy rate (CPR); secondary outcomes were live birth rate (LBR) and the cycle cancellation rate. Participants/materials, setting, methods True natural cycles (T-NC) and modified cycles with the use of triggered ovulation (M-NC) protocols for endometrial preparation were pursued in women with regular menses; otherwise, HRT was used. Immediate FET was defined when performed on the first menstruation following ovarian stimulation (i.e. withdrawal bleeding occurring after ovarian stimulation or after a failed fresh embryo transfer). Delayed FETs included those performed after at least two menstruations. Only the first transfer after ovarian stimulation was considered. Main results and the role of chance A total of 5,765 FET cycles were included in the study. FET cycles were achieved through T-NC in 4,477 cases (78% of all FETs), of which 364 were immediate and 4,113 were delayed. M-NC cycles accounted for 1% of the total amount of FETs and HRT protocols were pursued in 11% of cases. The analysis showed no impact of FET timing on CPR both in the whole cohort and in the subgroups based on different endometrial preparation protocols. Focusing on T-NC cycles, the CPR in the immediate group was comparable to that of the postponed group (29 vs. 32% respectively, OR 0.90 [95% CI 0.71 - 1.14]). On the other hand, LBR in T-NC cycles was significantly lower in the immediate group than in the delayed group (20% vs. 25% respectively, OR 0.74 [95% CI 0.56 - 0.96]). Consequently, immediate FETs resulted to be associated with a significantly higher rate of pregnancy loss (31% vs. 21% respectively, p = 0.03). Furthermore, cycle cancellation occurred more frequently in the immediate T-NC group, compared with the delayed group (18% vs. 12% respectively, p = 0.003). Similar results were observed in HRT cycles (15% in immediate FETs vs. 7% in delayed FETs, p = 0.08). Limitations, reasons for caution The retrospective nature of this study involves a risk of selection bias. Moreover, the reason for cycle cancellation was not always available; thus, these data were not included in the analysis. Lastly, these outcomes are only applicable to a similar cohort, that is Centers following a non-elective freeze-all policy. Wider implications of the findings This is the largest investigation on T-NC FETs and is among the very few studies providing separate results on different endometrial preparation protocols. Evidence from this study is in line with some previous retrospective analyses and in contrast with others. Thus, randomized controlled trials are needed to confirm these results. Trial registration number not applicable

P-414 Evaluation of small non-coding RNAs in uterine fluid in the diagnosis of endometrial receptivity in assisted reproductive technology programs

Abstract Study question To analyze endometrial receptivity in patients undergoing frozen-thawed embryo transfer (FET) based on the expression of small non-coding RNAs (sncRNAs): microRNAs and piwiRNAs. Summary answer According to our studies, the expression of sncRNAs in uterine aspirate may become an additional non-invasive and effective way to determine endometrial receptivity. What is known already Today, infertility remains an important medical and social problem. According to statistics, the effectiveness of ART programs remains at the level of 30-40%. Violation of the implantation mechanism is associated with a shift in the “implantation window” period and endometrial receptivity. Therefore, the search for an ideal marker of endometrial receptivity is a priority. Studying the expression of sncRNAs in endometrial secretions or uterine aspirate may become a potential marker of endometrial receptivity. Study design, size, duration The study included 102 infertile couples with women aged from 23 to 40 years. The patients were divided into two groups depending on the preparation of the endometrium for the FET: group 1 - 42 patients with natural cycle (NC); group 2 - 60 patients with hormone replacement therapy (HRT). SncRNAs were assessed in uterine aspiration on the day of FET. Participants/materials, setting, methods Uterine fluid was carried out in a volume of 5 to 50 μl with noninvasive technique. Next, the material was sent to the laboratory of applied transcriptomics for identification of sncRNAs. Quantitative assessment of sncRNAs in the endometrial secretion on the day of FET was carried out by deep sequencing on the NextSeq 500/550 platform (Illumina). Main results and the role of chance Logistic regression models were constructed for predicting pregnancy and endometrial receptivity based on the expression profiles of microRNAs and piwiRNAs in uterine aspirate, which are with 95% sensitivity and 74% specificity in the case of a combination of miR-34c-5p, miR-363–3p, miR-1180–3p, miR-361–3p, miR-183–5p (AUC = 0.91, p &amp;lt; 0.001) or with 74% sensitivity and 87% specificity in the case of a combination of molecules piR_004152, piR_020541, piR_001318, piR_014923, piR_017716 (AUC = 0.81, p &amp;lt; 0.001) determine the readiness of the endometrium for embryo implantation in the FET. In the NC and HRT groups, statistically significant differences were established between patients who became pregnant and patients with a negative result of the ART program in terms of baseline progesterone levels (p &amp;lt; 0.001). In the HRT group, positive correlations were revealed between endometrial thickness on the day of embryo transfer and the baseline level of progesterone (p = 0.04, r = 0.28). Limitations, reasons for caution The study was limited in sample size of patients and requires further randomized studies. Wider implications of the findings For the first time, a test system has been developed for assessing endometrial receptivity based on the expression of sncRNAs in patients both in the NC and in the HRT during the FET. It was found that increased initial progesterone levels correlate with positive outcomes of ART programs. Trial registration number not applicable

P-407 Exploring the effectiveness of tailoring estradiol supplementation duration to mimic the natural cycle length in artificial endometrial preparation for frozen embryo transfer cycles

Abstract Study question Is an estrogen supplementation duration similar in length to the follicular phase of a natural cycle associated with a better clinical pregnancy rate? Summary answer An estradiol supplementation duration, which is proportional to the natural follicular phase length, results in significantly better outcomes compared to shorter exposure durations What is known already The last decade witnessed a major increase in the use of single euploid frozen embryos. Artificial endometrial preparation (AEP) has proven to be effective to prepare the endometrium for implantation. However, several features of AEP need to be clarified mainly the duration of estrogen supplementation and until now no study took into consideration the difference between exogenous estrogen supplementation and the actual length of the follicular phase of each patient during AEP. Furthermore, most of the studies focused on upper limit of estrogen exposure while few considered the lower limit. Unfortunately, the results on pregnancy rates were mixed. Study design, size, duration 663 single euploid frozen embryo transfer cycles were included. Patients underwent vaginal ultrasound and hormonal blood measurement (Estradiol, Progesterone) on day 2 of their cycle. Estradiol valerate (4 mg the first 2 days then 6 mg daily, Orally) was started on the same day and another control was done around day 10. When triple lining pattern of the endometrium was seen, vaginal progesterone was initiated (800 mg daily) and SET was done after 120 hours. Participants/materials, setting, methods Patients with reported regular menstrual cycle (25-35 days) were included and the difference between estrogen supplementation and the actual follicular phase was calculated for each patient. Patients were sorted into 2 groups as follows: 231 women with normal interval (±2 days of follicular phase length), and 432 women with short interval (-3 days or shorter). Only the first single euploid embryo transfer cycles were analyzed for these patients. Main results and the role of chance Whereas patients had similar characteristics in both groups, the corresponding clinical pregnancy rates in normal and short supplementation groups were 69.3% (160/231) and 64% (277/432), respectively. After balancing the groups for BMI, embryo quality and endometrial thickness with inverse treatment probability weighting, those who received estradiol supplementation proportional to their natural follicular phase length had significantly higher clinical pregnancy rates compared to those who received it for shorter durations (adjusted OR: 1.48, 95% CI: 1.16 – 1.89, P = 0.002). Serum estradiol levels were available for 523 cycles, the clinical pregnancy rate in those within the 1st quartile (&amp;lt;155pmol/L), 2nd to 3rd quartile (155 to 275pmol/L) and 4th quartile (&amp;gt;275pmol/L) were 70%, 64.7% and 63%. After adjusting for BMI and embryo quality, higher (&amp;gt;275pmol/L) (adjusted OR: 0.84, 95% CI: 0.53-1.32, P = 0.442) and lower(&amp;lt;155pmol/L) (adjusted OR: 1.37, 95% CI: 0.85-2.22, P = 0.202) estradiol levels were not significantly associated with clinical pregnancy. Limitations, reasons for caution The present study could not address the influence of very long estrogen supplementation on clinical pregnancy rate due to limited samples. Further prospective studies, including larger populations, are needed to confirm our findings Wider implications of the findings Our data indicate that mimicking individually the natural cycle regarding estrogen supplementation duration in AEP for single euploid embryo transfer cycles can produce significant improvement in clinical pregnancy rates. Trial registration number not applicable

P-686 Optimal follicle size for triggering modified natural frozen embryo transfers

Abstract Study question What is the optimal follicle size threshold for performing trigger in modified natural cycle frozen embryo transfers (mNC-FET) cycles Summary answer The optimal follicle size threshold for performing trigger is 16 or 17, depending on the clinic’s preference to risk too early or too late triggering What is known already Recent studies reported higher fetal and maternal obstetrical complication rates are observed in artificial cycles compared to natural cycle frozen embryo transfers (NC-FET), making the natural cycle (NC) the preferred method. The modified natural cycle (mNC) provides a partial solution to overcome the disadvantages of the NC, mostly its repeated visits and inability to conveniently schedule the transfer. In a recently published large RCT, pregnancy rates between NC and mNC were similar, but many patients allocated to the mNC group had an LH surge prior to the hCG injection. Study design, size, duration A retrospective study consisting of 5,862 NC-FET was performed between 2018 and 2023 in a large tertiary hospital. Participants/materials, setting, methods Statistical analysis was conducted to determine the optimal follicle size threshold for triggering, for each threshold between 12-20, under the following assumptions: Clinic is closed on Sundays, ultrasound is initially performed on day 8 of the cycle and every 2 days afterwards, trigger is performed if the follicle size is at or above the threshold, unless clinic is closed 2 days after the current test day in which case no trigger was given. Main results and the role of chance The analysis measured 2 main outcomes for each trigger size threshold: The probability of triggering too early, defined as shifting the ovulation by 3 or more days. The probability of spontaneous LH surge to occur before triggering, possibly leading to transfer cancelation due to clinic closure on the designated transfer day. The traditional follicle size-based algorithm performed as follows: [Follicle size threshold]: ] Early trigger probability], [Closed day transfer probability] 12: 71.9%, 0.1% 13: 61.7%, 0.1% 14: 49.0%, 0.2% 15: 33.9%, 0.6% 16: 21.3%, 1.3% 17: 12.9%, 2.8% 18: 7.3%, 4.5% 19: 3.4%, 6.7% 20: 2.2%, 8.6% As can be seen from the results the optimal trigger threshold is either 16 or 17, depending on whether reducing transfers on days the clinic is closed has a higher importance than preventing a very early trigger day. Limitations, reasons for caution This was a retrospective study that performed statistical analysis to determine the probability of achieving each result based on the described logic. A prospective randomized clinical trial is warranted to validate the algorithm’s performance and consider variables that could not be accounted for, such as the patient’s endometrial width. Wider implications of the findings NC-FET protocols require the ability to adapt the transfer to clinics’ requirements. Given a traditional threshold-based methodology for triggering, a threshold of 16-17 is optimal depending on the risk preference of a clinic. Utilizing more advanced artificial intelligence algorithms based on ovulation prediction can significantly reduce both risk types. Trial registration number Not applicable

P-755 Effect of female age and specific cycle characteristics on success in modified natural frozen-thawed single euploid embryo transfer cycles (mNC/FET)

Abstract Study question Does the age of woman and specific cycle characteristics impact the success of modified natural frozen-thawed single euploid embryo transfer cycles (mNC/FET)? Summary answer The success rate of mNC/FET cycles is similar in all age groups. As the LH level increased ( above 23.5ng/dL) the success was significantly increased. What is known already The success of frozen embryo transfer can depend on various factors such as the patient’s age, the number of transferred embryos, the duration of embryo culture, and the method used to prepare the endometrium (Holschbach et al. 2023). However, there is currently insufficient evidence in the literature about the effect of age and cycle characteristics on the effectiveness of mNC/FET cycles in patients who have received single top and good quality euploid embryos. Study design, size, duration This study, conducted at Istanbul Memorial Hospital, ART and Reproductive Genetics Center, is a retrospective, single-centre analysis of 583 cycles of euploid mNC/FET in women aged 22-44 years. The study period was from July 2017 to January 2024. The cycles were divided into two groups based on the age of the women. Group A (n = 408) included women under the age of 38, while Group B (n = 175) included women aged 38 or above. Participants/materials, setting, methods The study investigated success predictors in mNC/FET with top and good-quality embryos. Excluded were women with recurrent pregnancy losses, recurrent implantation failure, high BMI (&amp;gt;35kg/m2), uterine/endometrial problems, severe male infertility, and autoimmune diseases. The mNC/FET success was a pregnancy lasting 12 weeks or more. The analysis included patient demographics and cycle outcomes. Chi-square, t-tests, and Mann-Whitney U tests assessed variables with a p-value &amp;lt; 0.005, indicating statistical significance. Main results and the role of chance No significant demographic differences were observed between the two groups, except for AMH levels. AMH levels were significantly higher in younger group than older group (3.02 ng/ml vs. 1.96 ng/ml, p = 0.000). The mNC/FET success rates were similar, with 76% in the younger group and 73% in the older group. Estradiol levels the day before trigger day were significantly higher in older group (278.02 pg/ml vs. 237.84pg/ml, p = 0.028). Both groups had similar mean LH levels (32.50 IU/L and 32.41 IU/L, respectively), estradiol levels and endometrial thickness on the trigger day. In the younger group, LH rise was observed later, follicular phase was longer, and embryo transfer day was later than the older group (p = 0.001). Univariate regression analysis revealed that increased LH level was an important independent variable in achieving mNC/FET success, with a 1.001-fold increase (p = 0.043). The LH levels of 23.5 IU/L or higher predicted high mNC/FET success with 60% sensitivity and 43% specificity based on the Receiver Operating Characteristic curve. (AUC: 0.606, %95 CI 0.540-0.671, p = 0.002). Prolonged infertility duration and previous ET failures reduced mNC/FET success by 0.889 and 0.544-fold, respectively (p = 0.002, p &amp;lt; 0.001). Higher β-hCG levels on days 14 and 16 increased mNC/FET success by 1.038-fold and 1.008-fold, respectively (p &amp;lt; 0.001). Limitations, reasons for caution Our study has limitations as it was retrospective. Prospective randomized studies are needed to evaluate the differences between the young (age &amp;lt;38) and old patient groups (age ≥38) based on mNC/FET success. The association between LH rise and mNC/FET success needs verification with a larger sample. Wider implications of the findings The mNC/FET success rates are similar in young (age &amp;lt;38) and old patient groups (age ≥38). However, prolonged infertility duration and previous failed ET cycles decreased the success of mNC/FET. Additionally, It is important to schedule trigger time after the optimum LH rise, which is 23.5 IU/L or higher. Trial registration number not applicable

O-149 The Endometriosis Longitudinal Fertility Study (ELFS): Outcomes for women with moderate or severe endometriosis who are trying to conceive

Abstract Study question What are the pregnancy outcomes for women with moderate or severe endometriosis who are trying to conceive? Summary answer Preliminary pregnancy rates in women with moderate/severe endometriosis are 11% for natural conception cycles and 22% for ART cycles. This is consistent with previous literature. What is known already Endometriosis is associated with reduced pregnancy rates in women attempting both spontaneous and assisted conception. Monthly fecundity is approximately half that of women without endometriosis and reduces further with increasing severity of disease. At present, the effect of surgery on the fertility outcomes of women with moderate or severe endometriosis remains unanswered. Treatment decisions are complex, particularly in those without pain symptoms seeking to optimise fertility outcomes. Study design, size, duration ELFS is a prospective multi-site longitudinal cohort study being conducted over 5-years. This interim report summarises data from August 2021-January 2024 in participants &amp;lt;38 years with evidence of in-situ moderate or severe endometriosis desiring fertility immediately or in the future. ELFS prospectively measures and compares monthly clinical pregnancy and live birth rates in women having either surgical or conservative management of endometriosis. This report assesses outcomes for those trying to conceive naturally or with ART. Participants/materials, setting, methods Following consent and baseline questionnaire completion, participants install the purpose-built ELFS App to their mobile phone or use a web-based option to complete cyclical surveys. The timing of surveys is dependent on a learned logic within the App and based on menstrual cycle length and pregnancy status. Participants report attempted conception cycles and any ART treatments undertaken with IVF or intrauterine insemination (IUI). Pregnancy outcomes are recorded as well as surgery during the study period. Main results and the role of chance There are 124 participants enrolled in ELFS, 86 (69%) are in the surgical cohort and 38 (31%) in the conservative cohort. There are 45 participants (36%) who have indicated they are actively trying to conceive during the study and a total of 171 cycles have been recorded from these participants. Of those trying to conceive, 24 (53%) have elected to have surgery during the study period, compared to 23 (29%) who were not trying to conceive. The study has captured 126 cycles with attempted conception. Of these, 42 cycles (33%) utilised ART compared to 84 cycles (67%) of natural conception attempts. Of the ART cycles, 27 (75%) involved IVF with fresh or frozen embryo transfer and there were 9 (15%) IUI cycles. To date, there are 25 reported pregnancies. The pregnancy rate for ART cycles was 22% (8/36) compared to 11% (9/87) in the natural conception cycles. Of those who conceived following surgery, 47% (8/17) of pregnancies were following ART and 53% (9/17) were natural conceptions. There are 2 live births recorded, both from the cohort who had surgery during the study. A total of 5 miscarriages (20%) have been reported, 4 (80%) from the surgical cohort. Limitations, reasons for caution In the absence of data to guide management in this area, clinician scope of practice and concurrent pain symptoms are likely to influence recommendations for ART or surgery in those trying to conceive with moderate or severe endometriosis. Wider implications of the findings This preliminary data is consistent with the current literature showing reduced natural and assisted conception rates in women with moderate or severe endometriosis. Long-term data will be required to determine if fertility and pregnancy outcomes are influenced by surgical management of endometriosis. Trial registration number ACTRN12621000431820 (ANZCTR)

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&amp;gt;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&amp;gt;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

P-700 Modelling individual follicle growth rates during ovarian stimulation in medically assisted reproduction cycles to enable prediction of final follicle profiles

Abstract Study question Can the growth of individual follicles during ovarian stimulation in medically assisted reproduction (MAR) cycles be modelled to enable prediction of final follicle profiles? Summary answer Mean follicle growth rate is 1.35 mm per day. Our model reliably forecasts follicle size profiles on the final scan within ±2mm with 75.1% accuracy. What is known already Follicle growth rates during ovarian stimulation are believed to be linear with various mean growth rates reported between 1 to 4 mm per day. Follicles are believed to grow faster during ovarian stimulation than in the natural menstrual cycle. The change in mean follicle size was 1.69 mm per day with ovarian stimulation in 131 MAR cycles, and 1.4 mm per day in 50 natural cycles. However, there are only very limited data using individual follicle sizes to model follicle growth to enable prediction of future follicle size profiles. Study design, size, duration We conducted a retrospective cohort study of 12,950 patients from 11 European clinics (2005-2023). First IVF/ICSI cycles with at least three follicles and two scans were analysed. Using 8,564 cycles with scans one or two days apart, 98,029 follicles were modelled to estimate follicle growth rates per day. Stratification by age, weight, total antral follicle count (AFC), initial FSH dose, and suppressant protocol was performed. Predictive modelling was conducted using 39,698 scans including 434,082 follicles. Participants/materials, setting, methods Assessment of Follicle Growth: Follicles were ranked and the difference in corresponding follicle sizes over consecutive scans estimated, and the impact of baseline characteristics evaluated. Prediction of follicle size profile on final scan: A kernel density estimator and random forest model were developed using the same training data. Both approaches estimated future follicle growth from the initial scan. The two predictions were combined using maximum-likelihood adjustments. Generalisation error was estimated using independent test data. Main results and the role of chance The mean growth rate of individual follicles was 1.350 mm per day (95% CI 1.346 – 1.353 mm per day), which was lower than the previously reported mean follicle growth rate during ovarian stimulation (1.7 mm per day). The large number of patient-cycles and follicles assessed enabled a precise estimate of follicle growth per day during ovarian stimulation. Age, total AFC, initial FSH dose, bodyweight, and suppressant protocol had no statistically significant effect on the estimated mean growth rate. Using only the follicle sizes from the initial scan during ovarian stimulation, our model had 75.1% accuracy for predicting follicle sizes on the final scan within ±2mm. Including follicle size data from the first two scans improved the accuracy of the model to predict the follicle sizes on the final scan to 80.4%. In a confirmatory analysis, we cross-validated our findings across each of 11 individual clinics, and similar estimates for the generalisation error as for the standard test/train split were maintained, suggesting suitable generalisability of these findings. Limitations, reasons for caution A key simplifying assumption in this study is that follicles demonstrate monotonic growth patterns. Specifically, the relative ordering of follicle sizes is preserved over the entire cycle. This assumption enables straightforward extraction of growth statistics from the observed data but assumes that follicles do not regress in size. Wider implications of the findings We have used a large high-quality dataset, allied to advanced modern machine learning techniques, to precisely estimate follicle growth rates during ovarian stimulation, and use this to reliably predict future follicle size profiles during ovarian stimulation. This supports the design, personalisation, streamlining and success of future assisted conception cycles. Trial registration number None

P-418 What is the optimal midluteal phase circulating progesterone level during a fresh IVF cycle? The answer differs from what one expects in a natural cycle

Abstract Study question Do midluteal phase circulating progesterone (P4) levels relate to the fresh embryo transfer clinical outcome and is rescue P4 useful in such cases? Summary answer Live birth rate (LBR) was significantly lower when P4 levels were below 20ng/ml and did not seem to benefit from rescue intramuscular P4. What is known already Preceding literature on the predictive value of midluteal P4 for LBR has shown conflicting results and frequently included small sample sets and heterogeneous ovarian stimulation protocols. The findings reported ranged from the lack of an association between P4 serum levels and clinical outcome to worse success rates seen either in the groups with the highest or the lowest P4 in midluteal phase. Furthermore, while rescue P4 has been empirically proposed, it has lacked formal assessment thus far. Study design, size, duration In total, 652 patients who underwent a fresh single blastocyst transfer in an IVF cycle in our center between 2014 and 2021 were included in this retrospective cohort study. All were GnRH-antagonist cycles using hCG trigger and luteal support with micronized vaginal progesterone 200mg twice daily. Rescue intramuscular progesterone was systematically administered whenever low midluteal P4 (below 1 standard deviation of the center’s mean) levels were detected. Participants/materials, setting, methods The cycles were subdivided into three groups according to the midluteal P4 levels: &amp;lt;10ng/ml, 10-20ng/ml and &amp;gt;20ng/ml. The primary outcome assessed was LBR, with secondary outcomes including clinical pregnancy (CPR) and pregnancy loss rates. Maternal age and body mass index (BMI), year of treatment, ovarian response, endometrial thickness, cause of infertility and embryo quality were controlled for using multivariable logistic regression. Main results and the role of chance The mean age ± standard deviation of patients was 36.8±3.8 years, while mean weight and BMI were 63.2±11.5kg and 23.3±3.9kg/m2, respectively. LBR in the group with P4&amp;gt;20ng/ml (46.5%) was significantly higher than in the groups with low and intermediate P4 serum levels (23.1% and 28.1%, respectively, p &amp;lt; 0.001). The same was true for CPR (54.5%, 30.8% and 37.9%, respectively, p &amp;lt; 0.001). Conversely, the rate of pregnancy loss in the group with P4&amp;gt;20ng/ml (23.8%) was significantly lower than in the groups with low and intermediate P4 levels (42.6% and 37.5%, respectively, p = 0.018). The multivariable logistic regression confirmed that, after controlling for the confounding variables described above, P4&amp;gt;20ng/ml remained significantly associated with a higher LBR (OR: 2.285; 95% CI: 1.506-3.469) and a lower pregnancy loss rate (OR: 0.488; 95% CI: 0.273-0.873), irrespective of the use of rescue IM P4. Finally, a multivariable linear regression analysis of the patients baseline characteristics suggested that the only parameter predictive of midluteal P4 levels was the female patients’ weight, with an inverse relationship (adjusted regression coefficient: -0,012; 95% CI: -0,016/-0,009). Limitations, reasons for caution As the pituitary suppression, ovulation trigger and luteal support protocols used in the cycles included in the study were homogeneous (in order to maximize the internal validity of the results), this limits the extrapolibility of the results to other stimulation protocols. Furthermore, we caution that this is a retrospective study. Wider implications of the findings The usefulness of both midluteal P4 serum measurements and rescue P4 should continue to be assessed. The reasons behind the need for higher levels of P4 in a fresh cycle, when compared to natural cycles, must also be investigated. Luteal support may require personalization according to female body weight. Trial registration number Not Applicable

P-692 Intra-subject variability of mid-luteal progesterone levels in natural/modified natural frozen embryo transfer cycles (NC/MNC): can we predict the need for luteal phase support?

Abstract Study question Are progesterone levels different in a NC versus a MNC? What is the predictive value of progesterone for the level in a next cycle? Summary answer Progesterone levels were not different in the mid-luteal phase of natural or modified natural cycles and only moderately agree with progesterone in a subsequent cycle. What is known already Progesterone is essential for successful implantation and pregnancy. Classical studies have shown considerable variability in progesterone levels during the luteal phase. More recently, progesterone levels during the mid-luteal phase have regained interest, in order to improve chances of pregnancy in frozen embryo transfer cycles. Several studies have suggested that if progesterone on the day of embryo transfer are below a certain cut off level, progesterone production would not be sufficient and luteal support should be provided. Study design, size, duration This was a prospective observational study at a tertiary fertility centre in Belgium, performed between January 2022 until March 2023. Serum progesterone on the day of frozen embryo transfer was measured in all patients undergoing frozen embryo transfer in a natural (NC) or modified natural cycle (MNC). Patients were eligible for the current study if at least two progesterone levels in two frozen embryo transfer cycles were available. Participants/materials, setting, methods A linear mixed model was used to calculate systematic differences in progesterone levels between NC and MNC. The intra-subject agreement was quantified by means of the intra-class correlation coefficient (ICC) with 95% confidence interval. The ICC ranges between 0 and 1, with high values (&amp;gt; 0.75) indicating a good to excellent level of agreement between measurements taken in the same subject. Progesterone levels were measured with COBAS Elecsys ECLIA progesteron assay by Roche Diagnostics. Main results and the role of chance All women were between 18 and 40 years of age. Forty patients with at least two frozen embryo transfers in NC and in MNC were included, i.e. 87 cycles. Their mean age was 32 (26-42) years, the mean BMI was 23.1 (16.9-34.5) kg/m2. Mean progesterone levels on the day of transfer were 13.0 ng/ml (11.0-15.0) in NC and 13.8 ng/ml (12.0-15.6) in MNC. These levels were not systematically different (P = 0.47). The intra-subject agreement showed an ICC of 0.66 in NC and 0.61 in MNC. Hence, our data indicate that progesterone levels in the mid-luteal phase of a NC are similar to those in a MNC. However, progesterone levels in one cycle only moderately agree with those in a next cycle. Limitations, reasons for caution Our data were obtained from women who had at least two frozen embryo transfer cycles and therefore may include progesterone levels different from those in good prognosis patients who got pregnant after a fresh embryo transfer or after a first frozen embryo. Wider implications of the findings Our data indicate that the moderate agreement of progesterone between cycles would imply assessment of progesterone in each cycle. In daily clinical practice, luteal phase support should be considered once a single progesterone measurement is below earlier described cut off levels predictive of ongoing pregnancy after frozen embryo transfer. Trial registration number not applicable

P-572 Circadian serum progesterone variations on the day of frozen embryo transfer in a modified natural cycle protocol

Abstract Study question Does a circadian variation of serum progesterone (P) on the frozen embryo transfer (FET) day in a modified natural cycle (mNC) exist? Summary answer A statistically significant diurnal variation of serum P on the day of a FET in a mNC protocol exists. What is known already In recent years, the proportion of FET cycles has increased dramatically. To further optimize pregnancy outcomes after FET, recent studies have focused on serum luteal P levels in both natural and artificially prepared FET cycles. Despite the different cut-off values proposed to define low serum P in the natural cycle, it is generally accepted that lower serum P values (&amp;lt;10ng/ml) around the day of FET are associated with negative reproductive outcomes. However, a single serum P measurement on the day of FET is not reliable given that P levels are prone to diurnal fluctuations and are impacted by patients’ characteristics. Study design, size, duration A prospective cohort study was conducted in a single fertility center including 22 patients performing a single blastocyst mNC-FET from August 2022 to August 2023.Serum P levels were measured on the day of transfer at 08:00h,12:00h,16:00h and 20:00h.Differences between P levels were compared using the Wilcoxon signed-rank test.The sample size was calculated to detect a difference of 15% between the first and last P measurements with a 5% false-positive rate and a 95% confidence interval. Participants/materials, setting, methods Patients with a normal BMI, between 18 and 40 years old, without uterine diseases were eligible. Patients utilizing donated oocytes were excluded. The mNC-FET protocol implied monitoring the normal ovarian cycle and triggering ovulation with an injection of 250 mcg of choriogonadotropin alfa when a pre-ovulatory follicle (16-20 mm diameter)was visualized. The blastocyst was transferred seven days later. The patients were not supplemented with exogenous P at any moment before the day of the FET. Main results and the role of chance The mean age and BMI of the study population were 33.6 ± 3.8 years and 22.7 ± 1.8 kg/m2, respectively. Mean P values at 08:00h,12:00h,16:00h, and 20:00h were 14.6 ± 4.5ng/ml, 14.7 ± 4.1ng/ml, 12.9 ± 3.5ng/ml, and 14.6 ± 4.3ng/ml, respectively. The mean P levels at 16:00h were statistically significantly lower compared to all other time points (p &amp;lt; 0.05:p= 0.007 between P at 8:00h and 16:00h;p = 0.003 between P at 12:00h and 16:00h;p=0.007 between P at 16:00h and 20:00h). Absence of a statistically significant fluctuation was observed between P values at the other time points (p = 0.88 between P at 8:00h and 12:00h;p=0.96 between P at 8:00h and 20:00h;p=0.83 between P at 12:00h and 20:00h). This study confirms the existence of an intra-day variation of serum P on the day of mNC-FET, with lower values at 16:00h. This highlights the importance of standardizing P measurement on the day of FET; we propose this should always be assessed in the morning to be more reliable and to allow the time to prescribe additional exogenous progesterone if lower P values (&amp;lt;10ng/ml) are detected. This standardization would allow fertility clinics around the world to compare their results and develop internationally recognized treatment protocols. Limitations, reasons for caution The study’s main limitation include the small sample size that may cause a bias when results are extrapolated to a larger subfertile population undergoing mNC-FET. Ideally, larger prospective trials including a more heterogeneous patient population would be necessary to validate our findings. Wider implications of the findings This study demonstrates the existence of a diurnal fluctuation of serum P on the mNC-FET day highlighting the importance of a standardized time point for its measurement.This is important when clinical actions, like additional exogenous P supplementation, are considered when encountering P values lower than 10ng/ml on the FET day. Trial registration number NCT05511272

P-723 Egg-conomized frozen embryo transfer

Abstract Study question Is home-based monitoring of ovulation cost effective compared with hospital-controlled ovulation in women undergoing frozen embryo transfer in the natural cycle? Summary answer Home-based monitoring to time frozen-embryo-transfer is cost-effective compared to hospital-controlled ovulation, as it results in lower cost with no difference in ongoing pregnancy rates. What is known already Women undergoing frozen embryo transfer treated by home-based monitoring of ovulation or hospital-controlled ovulation have comparable ongoing pregnancy rates. We found no differences in secondary outcomes, including the risk of undetected ovulation. Complete home-based monitoring of ovulation with LH hormone kits is feasible and requires no hospital visits at lower costs since ultrasound monitoring and hCG injection are no longer needed. Both consequences imply potentially lower costs after home-based monitoring of ovulation. Study design, size, duration We performed an economic evaluation of home-based monitoring of ovulation compared with hospital-controlled ovulation in a national multicentre randomised controlled trial in women undergoing frozen embryo transfer (ANTARCTICA-2 RCT Dutch TrialRegister Trial NL6414). 1464 women were randomly assigned between April 10, 2018, and April 13, 2022, with 732 allocated to home-based monitoring and 732 to hospital-controlled monitoring. All randomized patients were included according to the intention-to-treat principle. Participants/materials, setting, methods We performed a cost-effectiveness analysis from a hospital and societal perspective. We compared the direct medical costs of home-based monitoring of ovulation and hospital-controlled ovulation during one cycle until an ongoing pregnancy occurred. Direct medical costs included digital ovulation tests, transvaginal ultrasound monitoring of the dominant follicle followed by hCG trigger, hormone measurements in blood, treatment for miscarriage or ectopic pregnancy. Nonparametric bootstrapping was used to calculate cost estimates and differences with 95% confidence intervals. Main results and the role of chance Ongoing pregnancy rates were 20·8% in the home-based monitoring group and 20·9% in the hospital-controlled group (risk ratio [RR] 0·99 [90% CI 0·81 to 1·22]; risk difference [RD] -0·14 [90% CI -3·63 to 3·36]). No differences in ectopic pregnancies or miscarriages were found. Home-based monitoring required less travelling to the hospital, less ultrasounds by doctors or nurses, less hormone measurements in blood, and less medication (hCG injections). Mean direct medical costs per woman in the home-based monitoring of ovulation group and in the hospital-controlled ovulation group were €752 versus €991, respectively, with a mean difference of €239. The ICER consistently showed home-based monitoring to be less costly with no difference in effectiveness. Consequently, the cost difference from a societal perspective was larger. The mean costs per woman in the home-based monitoring of ovulation group and in the hospital-controlled ovulation group were €787 versus €1395, respectively, with a mean difference of €607 in favour of home-based monitoring. Limitations, reasons for caution We report on one cycle per women. Women usually undergomultiple frozen-embryo-cycles. This is of importance for clinical decision making aboutimplementation of home-based monitoring, since the cumulative benefit of multiple cycles perwoman should be taken into account. Wider implications of the findings The results of our trial show that home-based monitoring time frozen embryo transfer is equally effective but less costly compared with hospital-controlled ovulation. In this era of climate change and shortages of hospital staff and waiting lists, any process that takes less human labour is important. Trial registration number Dutch Trial Register (Trial NL6414)