Outcomes of FET
The primary outcomes after FET were pregnancy outcomes consisting of biochemical pregnancy, clinical pregnancy (CP), ectopic pregnancy, abortion, premature delivery and live birth (LB). A biochemical pregnancy was indicated by a serum human chorionic gonadotropin (HCG) level ≥ 10 IU/L on day 14 after ET. A clinical pregnancy was considered detecting the presence of a gestational sac by ultrasonography on day 35 after ET. A diagnosis of ectopic pregnancy occurred when a developing blastocyst was found implanted outside the endometrial cavity. Abortion was defined as a clinical pregnancy lost before 28 weeks’ gestation. Premature delivery was regarded as neonatal birth from the 28th to 37th week of gestation. Live birth was considered as the delivery of any viable neonate at 28 weeks of gestation or later.