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

Abstract Background To evaluate whether progesterone-induced changes in endometrial thickness (EMT) affect singleton infant outcomes during frozen-thawed embryo transfer (FET) cycles. Methods This 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 in