Study design and patients
This retrospective cohort study includes 821 women with euploid embryo who underwent frozen-thawed embryo transfer between January 2012 and March 2020 in the Center for Assisted Reproductive Technology of Northwest Women’s and Children’s Hospital, China. All the patients were followed-up for one year after embryo transfer. This study was approved by the ethics committee of the hospital (number 2019013). Because of the retrospective character of the study, informed consent was waived. Patient data were anonymized. Data was extracted from electronic medical record system. Inclusion criteria were: first transfer cycle of a single frozen-thawed, euploid blastocyst embryo after IVF with preimplantation genetic testing for aneuploidies (PGT-A). Exclusion criteria were multiple FET cycles. Patients were included a single time in the study. BMI was calculated as weight divided by squared height. Women were considered underweight if BMI was <18.5 kg/m2, normal weight if 18.5 kg/m2 ≤ BMI <25 kg/m2, overweight if 25 kg/m2 ≤ BMI <30 kg/m2, and obese if BMI was ≥30 kg/m2 according to the most recent BMI classification by the World Health Organization (WHO) [8].