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.