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

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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 (<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 < 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 (<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