Introduction
As an important technique in assisted reproductive technology (ART), embryo transfer is widely adopted in clinical practice. As a complementary treatment for fresh embryo transfer, in FET, available embryos are frozen, and then the appropriate time for their transplantation can be selected. Compared with fresh embryo transfer, FET has advantages of reduced trauma from repeated ovulation induction and egg aspiration as well as reduced costs for patients. However, its pregnancy outcomes are of great concern. Although some existing studies point out that FET is not associated with a risk of preeclampsia and is helpful in reducing the incidence of premature birth, low birth weight infants and small for gestational age infants, others argue that FET significantly increases the preeclampsia risk.1-4
The placenta is an important organ that connects the mother and the foetus during pregnancy. The formation of the placenta involves the invasion of trophoblast cells and the remodelling of uterine spiral arteries, which can lead to various adverse pregnancy complications. ART techniques, such as the application of ovulation-inducing drugs, in vitro embryo culture, freezing technology, and corpus luteum support during the transplantation cycle, may influence the formation and function of the placenta, lead to structural abnormalities in placental villi and vascular and inflammatory changes, and eventually affect pregnancy outcomes.
Therefore, understanding the potential impact of different embryo transfer techniques on placenta formation is a guide for clinicians in selecting the appropriate embryo transfer protocol and strengthening the management of pregnancy after successful transfer to minimize pregnancy complications. The aim of this retrospective study was to analyse cases of live births after fresh embryo transfer and frozen-thawed embryo transfer with different endometrial preparation protocols over a 10-year period at our institution and their pregnancy outcomes and placental pathological characteristics.