Frozen embryo transfer (ET) has been associated with better perinatal and obstetric outcomes than fresh embryo transfer (ET). 1 This advantage of frozen ET over fresh ET, along with the advent of single embryo transfer, has consequently caused a substantial increase in the use of frozen ET and elective freezing. 2 Few observational studies, however, have highlighted the higher risk of hypertensive disorders in pregnancy (HDP) after frozen ET compared with both natural conception and fresh ET.
3,4 A recent meta-analysis of three randomized trials showed an increased risk of HDP after elective freezing than after fresh ET. 5 Studies involving comparison with natural conception are needed to account for potential contribution from infertility. Additionally, the sibship designs can be used to control the unmeasured or unknown parental factors such as genetics, preconception lifestyle,…
