Study design
The 9832 FET cycles were divided into three groups according to female age: < 35, 35-39 and > 39 years, and two groups depending on their post-thawed culture period: short culture (2-3 h) group (S) and long culture (18-20 h) group (L). The long culture group divided into three groups depending on the blastomere growth number: ≤ 2 group (blastomere growth number of each transferred embryo was ≤ 2), one ≤ 2 and the other > 2 group (blastomere growth number of the two transferred embryos was that one was ≤ 2 and the other was greater than > 2) and > 2 group (blastomere growth number of each transferred embryo was > 2).
Clinical outcomes included implantation rate (IR), clinical pregnancy rate (CPR), multiple pregnancy rate (MPR), abortion rate (AR), live birth rate (LBR), ectopic pregnancy rate (EPR), gestational age, preterm labor rate, mode of delivery, sex ratio, congenital malformation, low birthweight rate and birthweight were compared. IR was defined as the number of gestational sacs on ultrasound divided into the total number of transferred blastocysts. Clinical pregnancy was considered as a gestational sac seen by vaginal ultrasound scan by 6-7 weeks of gestation, while multiple pregnancy was described as having more than one gestational sac. Meanwhile, abortion was the spontaneous cessation of a clinical pregnancy, and live birth was defined as delivery of a live fetus at gestational age ≥ 24 weeks.