Study design
With the approval of the Ethics Committee of the Third Hospital of Peking University (No. 2020-419-01), the clinical data of pregnant women who underwent in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) and embryo transfer and delivered at the Third Hospital of Peking University from January 2012 to February 2022 were collected. The live birth cases were divided into three groups according to the embryo transfer method: the fresh embryo transfer group, natural cycle (NC)-frozen-thawed embryo transfer (FET) group, and hormone replacement therapy (HRT)-FET group.
Inclusion criteria: gestational age ≥24 weeks, live birth, and single pregnancy or single intrauterine foetal demise or reduction before 12 weeks.
Exclusion criteria: 1) ART with donor sperm , 2) conception after preimplantation genetic diagnosis, 3) definite foetal malformation before delivery, and 4) intrauterine foetal death or reduction after12 weeks.