Main finding
From January 2012 to February 2022, a total of 3,920 cases were collected, including 1822 fresh embryo transfer cases, 1353 NC-FET cases and 745 HRT-FET cases. The HRT-FET group had the highest risk of preeclampsia, cervical insufficiency and postpartum haemorrhage among the three groups.The birth weight of neonates was higher in the FET group than that of neonates in the fresh embryo transfer group. However, the incidence of placental histological abnormalities in the fresh embryo transfer, NC-FET, and HRT-FET groups was similar. The placental pathological analysis of the submitted placentas showed that there was no significant difference in the incidence of placental infarction, calcification, syncytiotrophoblastic nodule hyperplasia or fibrin deposition.