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.