This retrospective study was approved by the Ethics Committee of the Third Affiliated Hospital of Zhengzhou University. All patients who underwent in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) at the reproduction centre of the Third Affiliated Hospital of Zhengzhou University between January 2012 and December 2015 were included in this study. Patients who underwent transfer of one or two cleavage-stage embryos were included in our study. We divided these patients into two groups: group A comprised cycles in which one poor-quality embryo was transferred with one good-quality embryo, and group B comprised cycles in which two good-quality embryos were transferred. The outcomes we analysed were the implantation rate, the clinical pregnancy rate, the ectopic pregnancy rate, the abortion rate, the live birth rate, the number of foetuses, birthweight, gestational age, and the incidence of low birth weight and preterm birth.
Clinical pregnancy was diagnosed by transvaginal ultrasound of at least one gestational sac or definitive clinical
signs of pregnancy. Live birth was defined as one or more new-borns after 20 completed weeks of gestation. Ectopic
pregnancy was defined as a pregnancy that did not occur inside the uterine cavity. Low birth weight was defined as
a newborn weight less than 2500 g. Preterm birth was defined as a gestational age of less than 37 weeks.
Embryo quality Embryo quality was determined according to a ranking system. Class I embryos were defined as those with six to ten cells on day 3 and < 5% embryo fragmentation. Class II embryos were defined as those with six to ten cells on day 3 and 5–20% embryo fragmentation. Class III embryos were defined as those with uneven cell size, irregular cell morphology, and embryo fragmentation from 21% to 49%. Class IV embryos were defined as those with extremely uneven cell size, a large number of intracellular vacuoles, arrest of embryonic development or embryo fragmentation of over 50%. Class I embryos and Class II embryos were defined as good-quality embryos.