Study design
The 6096 frozen-thawed blastocyst transfer cycles were divided into five groups based on number and quality: a good-quality blastocyst (G), two good-quality blastocysts (GG), a good-quality blastocyst and a poor-quality blastocyst (GP), a poor-quality blastocyst (P), and two poor-quality blastocysts (PP). The patients were subsequently divided into three groups according to female age: < 35, 35-39 and > 39 years. Pregnancy outcomes included implantation rate (IR), clinical pregnancy rate (CPR), multiple pregnancy rate (MPR), abortion rate (AR), live birth rate (LBR), gestational age, mode of delivery, sex ratio, congenital malformation and birthweight. IR was defined as the number of gestational sacs on ultrasound over the total number of transferred blastocysts. Clinical pregnancy was defined as a gestational sac seen by vaginal ultrasound scan by 6-7 weeks of gestation, while multiple pregnancy was defined as having more than one gestational sac. Meanwhile, abortion was defined as the spontaneous cessation of a clinical pregnancy, and live birth was defined as delivery of a live fetus at gestational age ≥ 24 weeks.