Progesterone-induced endometrial thickness changes do not affect singleton neonatal outcomes in frozen embryo transfer: a retrospective analysis of 6,331 newborns.

BackgroundTo evaluate whether progesterone-induced changes in endometrial thickness (EMT) affect singleton infant outcomes during frozen-thawed embryo transfer (FET) cycles.MethodsThis retrospective study analyzed 6,331 singleton live births following frozen-thawed Day 3 embryo transfers. EMT was measured via transvaginal ultrasound one day before progesterone initiation and on the day of FET. Participants were grouped by EMT change: increase, decrease, or stable. Primary outcomes included mean birthweight, low birthweight (LBW), and small-for-gestational-age (SGA) status. Associations between EMT changes and neonatal outcomes were evaluated using multivariable linear and logistic regression.ResultsNo significant differences in mean birthweight were observed among the three groups (3,355.30 ± 502.69 g vs. 3,351.30 ± 474.79 g vs. 3,344.26 ± 514.54 g; p = 0.753). Compared with the EMT stable group, the EMT decreased group showed no significant increase in LBW (1.1%; aOR 1.645, 95% CI 0.818-3.307) or SGA (2.7%; aOR 1.141, 95% CI 0.783-1.662). Similarly, the EMT increased group showed no significant association with LBW (aOR 1.310, 95% CI 0.723-2.375) or SGA (aOR 0.912, 95% CI 0.660-1.261). Multiple linear regression confirmed that gestational age and infant gender significantly influenced birthweight, while EMT change did not.ConclusionsEMT may increase, decrease, or remain stable after progesterone administration in FET cycles. However, these changes do not demonstrate an independent association with adverse perinatal outcomes in term singleton infants.