Abstract Objective: To analyse the impacts of fresh embryo transfer and frozen‒thawed embryo transfer cycles with different endometrial preparation protocols on clinical outcomes and placental pathology. Design: Retrospective case‒control study. Setting: Peking University Third Hospital. Population: A total of 3920 Single live birth cases after in vitro fertilization/intracytoplasmic sperm injection and embryo transfer cycles. Method: Cases were divided into the fresh embryo transfer, natural cycle (NC)-frozen-thawed embryo transfer and hormone replacement therapy (HRT)-FET groups, and clinical outcomes and placental pathology characteristics were compared. Main Outcome Measures: preterm birth, preeclampsia, postpartum haemorrhage, placenta implantation, placenta previa, placental accreta, cervical insufficiency, neonatal weight and placental pathology. Result:The risks of preeclampsia, postpartum haemorrhage and preterm birth were significantly higher in the HRT-FET group than the fresh embryo transfer and NC-FET groups (13.42% vs 5.49% vs 5.91%, 21.7% vs 12.1% vs 11.0%, 10.5% vs 7.7% vs 7.6%, p < 0.05). Birth weight was lower in the fresh embryo transfer group than the NC-FET and HRT-FET groups (p<0.05). There was no statistically significant difference in the incidence of placental structural abnormalities and pathological characteristics among the groups. Conclusion: HRT-FET cycles were associated with increased maternal and foetal complications compared to fresh embryo transfer and NC-HRT cycles. There was no significant difference in the occurrence of placental structural abnormalities or pathological changes among the transfer methods. Keywords: Fresh embryo transfer; Frozen-thawed embryo transfer; Endometrial preparation protocol; Clinical outcome; Placenta Tweetable abstract: Different Embryo Transfer techniques influence the clinical outcomes but not placental pathology .