Pregnancy outcomes
The primary pregnancy outcome was the cumulative live birth (CLB), defined as at least one liveborn baby at ≥ 20 weeks gestation resulting from an ART-initiated cycle, including all fresh and subsequent FET cycles, until one live birth occurred or all embryos were used.
Other pregnancy outcomes assessed in the study included implantation rate, CPR, ongoing pregnancy rate (OPR), LBR, ectopic pregnancy rate, biochemical pregnancy rate, early spontaneous abortion rate (ESAR), late spontaneous abortion rate (LSAR), singleton pregnancy rate, and multiple pregnancy rate per fresh embryo transfer cycle. Implantation rate was determined based on the number of gestation­al sacs detected by ultrasound scan 5–7 weeks after embryo transfer divided by the number of embry­os transferred. Clinical pregnancy was defined as the presence of a gestational sac with observed fetal heart by ultrasound 5 weeks after embryo transfer, while ongoing pregnancy was defined as a pregnancy with fetal heart activity detected by ultrasound after 12 weeks of ges­tation. Live birth was defined as at least one liveborn baby at ≥ 20 weeks gestation with fresh embryo transfer. Early spontaneous abortion was defined as miscarriage occurring during the first trimester of pregnancy, while late spontaneous abortion was defined as miscarriage occurring after the first trimester of pregnancy. Multiple pregnancy was defined as more than one gestation­al sacs detected by ultrasound scan 5–7 weeks after embryo transfer.
Perinatal outcomes, obstetric complications, andcongenital defects
Obstetric and perinatal complications, as well as congenital defects of deliveries from an ART stimulation cycle, including all fresh and subsequent FET cycles, were completely followed up. In this study, live births after fresh embryo transfer were referred to as fresh live births (FLBs); live births from all fresh and subsequent FET cycles were called CLBs.
Obstetric complications included in this study were HDP, GDM, pre-eclampsia, preterm premature rupture of membranes, placenta previa, polyhydramnios, oligohydramnios, postpartum hemorrhage, placental abruption, and placenta accreta.
Perinatal outcomes included mode of delivery, gestational age, weight at birth, neonatal asphyxia and infection, neonatal intensive care unit (NICU) admission, and early neonatal death. Specifically, very preterm birth and preterm birth were defined as a live birth or stillbirth occurring before 32 and 37 gestation weeks, respectively. Very low birth weight and low birth weight were respectively defined as birth weight lower than 1,500 grams and 2,500 grams. Fetal macrosomia was defined as birth weight > 4,000 grams. Early neonatal death was defined as the death of a liveborn baby within 7 days of birth.
Based on the International Statistical Classification of Diseases Q codes (Q00-Q99)18, 10th Revision, congenital malformations followed up in this study included cleft lip and cleft palate and congenital malformations originating from the nervous, circulatory, digestive, urogenital, and musculoskeletal systems.