Spontaneous ovulation, hormonal profiles, and the impact of progesterone timing variation on outcomes in natural proliferative phase frozen embryo transfer cycles with single euploid blastocyst transfer.

BACKGROUND

Natural cycle frozen embryo transfer (NC-FET) lowers obstetric risks by preserving ovulation and corpus luteum but limits scheduling flexibility. Natural proliferative phase FET (NPP-FET) offers a scheduling-friendly alternative, assuming ovulation is maintained after flexible progesterone (P4) initiation during the follicular phase. Only three peer-reviewed studies have investigated NPP-FET protocols, yet none verified spontaneous ovulation, characterized hormonal dynamics, or evaluated whether variation in P4 initiation timing influences clinical outcomes. Preserving spontaneous ovulation is essential for NPP-FET to replicate the physiologic benefits of NC-FET; confirming its consistency is critical to validating NPP-FET as a viable protocol. To our knowledge, this is the first study to comprehensively address these gaps, providing novel evidence to support NPP-FET's clinical feasibility.

METHODS

This retrospective cohort study included 196 first-time NPP-FET cycles with single euploid blastocyst transfers between January 2023 and October 2024. Dydrogesterone (40 mg/day) was initiated upon meeting the following criteria: leading follicle ≥ 14 mm, endometrial thickness ≥ 7 mm, serum estradiol > 150 pg/mL, and P4 < 1.5 ng/mL. Ultrasound and hormonal monitoring continued until ultrasound-documented ovulation (UDO), followed by three days of hormone assessments. Ovulation was confirmed by UDO and serum P4 > 3.0 ng/mL. Embryo transfer occurred on day 6 of dydrogesterone exposure. Multivariable logistic regression evaluated associations between pregnancy outcomes and P4 timing-related variables, including follicular phase duration, estradiol and follicular diameter at P4 initiation, P4 start-to-UDO interval, UDO-to-FET interval, and serum P4 on FET day.

RESULTS

Spontaneous ovulation was confirmed in all participants. Median follicular diameter one day before UDO was 18.6 mm. UDO occurred within 1-2 days in 96.4% and 92.2% of cases based on two LH surge criteria. Peri-ovulatory hormone profiles resembled natural cycles. Clinical pregnancy, ongoing pregnancy, and clinical loss rates were 66.3%, 58.7%, and 11.5%, respectively. Embryo morphology and biopsy day predicted pregnancy outcomes, while P4 timing-related variables showed no association.

CONCLUSIONS

Flexible dydrogesterone initiation at follicular diameters ≥ 14 mm, based on predefined criteria, preserves spontaneous ovulation and natural hormonal dynamics. Pregnancy outcomes were consistent across P4 initiation timings, supporting NPP-FET as a clinically viable, physiologically grounded, and scheduling-friendly protocol.