Serum estradiol level on trigger day impacts clinical pregnancy rate in modified natural frozen embryo transfer cycles.

Objective To examine the predictive value of serum estradiol and progesterone on the day of human chorionic gonadotropin (hCG) administration and embryo transfer for clinical pregnancy rate in modified natural-cycle frozen embryo transfer (NC-FET). Methods In a longitudinal prospective study, all eligible women who underwent NC-FET cycles with hCG triggering in Royan Institute, Tehran, Iran, from June 1, 2015, to December 31, 2016, were evaluated. Serum estradiol and progesterone levels were measured at menstrual cycle initiation, on day of trigger with hCG, on day of embryo transfer, and in pregnant women every 7 days until the observation of a gestational sac with embryonic heartbeat. Results In total, 101 modified natural FET cycles were assessed, and the clinical pregnancy and live birth rates achieved were 34 (33.6%) and 32 (31.6%), respectively. The changes in estradiol level during early pregnancy showed an increase by an average of 200 pg/mL per week. Multivariable logistic regression analysis showed that only the estradiol level on the hCG day was a significant predictive variable for clinical pregnancy following NC-FET (P=0.04). Conclusion Estradiol level on the day of hCG trigger predicted the clinical pregnancy rates after modified NC-FET; this likely mirrored the developmental competence of the corpus luteum and an appropriate luteal structure-function.