Impact of endometrial thickness alteration in response to progesterone administration on neonatal outcomes in frozen embryo transfer cycles: analysis of 6331 singleton newborns

Background To assess the impact of progesterone-induced changes in endometrial thickness (EMT) on singleton infant outcomes during frozen-thawed embryo transfer (FET) cycles. Methods This retrospective observational study included a total of 6331 singleton live births resulting from frozen-thawed Day 3 embryo transfer. Endometrial thickness (EMT) was assessed using transvaginal ultrasound one day prior to progesterone administration and on the day of frozen embryo transfer (FET) to examine any variations in EMT. The study population comprised 6331 women, who were categorized into three groups based on changes in EMT: the EMT increase group, the EMT decrease group, and the EMT stable group. The primary outcomes investigated in this study were mean birthweight, low birthweight (LBW), and small-for-gestational age (SGA). A multivariable linear regression analysis was performed to explore the association between changes in EMT following progesterone administration and newborns' birthweight. Results Despite any fluctuations in EMT levels on the day of FET compared to one day prior to progesterone administration, there were no statistically significant differences observed in the absolute mean birthweight of singletons among the three groups (3355.30 ± 502.69 vs. 3351.30 ± 474.79 vs. 3344.26 ± 514.54, P = 0.753). In comparison to the stable EMT group, the decreased EMT group had incidences of LBW and SGA in term infants at 1.1% (adjusted odds ratio [aOR]: 1.645, 95% confidence interval [CI]: 0.818–3.307) and 2.7% (aOR: 1.141, 95% CI:0.783–1.662), respectively; however, there was no significant association between the increased EMT group and risks of LBW (aOR: 1.310, 95% CI:0.723–2.375) or SGA (aOR:0.912, 95% CI:0.660–1.261). The multiple linear regression analysis revealed that both gestational age and infant gender exerted significant influences on singleton birthweight, while any alteration in endometrial thickness subsequent to progesterone administration did not yield a statistically significant impact on singleton birthweight. Conclusions The extent of EMT may exhibit variability, either increasing, decreasing, or remaining stable on the day of frozen embryo transfer (FET) compared to one day prior to progesterone administration. However, it is important to note that changes in EMT following progesterone administration do not demonstrate an independent association with adverse perinatal outcomes in term infants during FET cycles.