Increased incidence of post-term delivery and Cesarean section after frozen-thawed embryo transfer during a hormone replacement cycle.

Purpose: This study aimed to clarify the risks of adverse pregnancy outcomes in patients who conceive singletons after frozen embryo transfer (FET) during a hormone replacement cycle and their offspring.

Methods: A retrospective cohort study was conducted in patients who conceived after FET, based on the Japanese-assisted reproductive technology registry for 2013. The perinatal outcomes in cases with live-born singletons achieved through natural ovulatory cycle FET (NC-FET) (n = 6287) or hormone replacement cycle FET (HRC-FET) (n = 10,235) were compared. Multiple logistic regression analyses were performed to determine the potential confounding factors.

Results: The frequencies of macrosomia (1.1% in NC-FET and 1.4% in HRC-FET; P = 0.058) were comparable between patients after NC-FET and HRC-FET. The proportions of post-term delivery (0.2% in NC-FET and 1.3% in HRC-FET; P < 0.001) and Cesarean section (33.6% in NC-FET and 43.0% in HRC-FET; P < 0.001) were higher in patients after HRC-FET than in patients after NC-FET. The risks of post-term delivery (adjusted odds ratio (AOR) 5.68, 95% confidence interval (CI) 3.30-9.80) and Cesarean section (AOR 1.64, 95% CI 1.52-1.76) were also higher in patients after HRC-FET than in patients after NC-FET.

Conclusions: Patients who conceived singletons after HRC-FET were at increased risk of post-term delivery and Cesarean section compared with those who conceived after NC-FET.