Assisted reproductive technology-associated risk factors for retained products of conception.

Objective: To evaluate assisted reproductive technology-associated risk factors for retained products of conception among live births.

Design: Registry-based retrospective cohort study.

Setting: Not applicable.

Patient(s): Cycle-specific data for a total of 369,608 singleton live births after fresh and frozen-thawed embryo transfers (FETs) between 2007 and 2017 were obtained from the Japanese assisted reproductive technology registry.

Intervention(s): None.

Main Outcome Measure(s): Retained products of conception after delivery. Odds ratios and 95% confidence intervals for risk factors associated with retained products of conception during fresh and frozen cycles.

Result(s): In total, 132 deliveries (0.04% of eligible assisted reproductive technology registry deliveries) had retained products of conception; 122 (92.4%) of these deliveries occurred after FET transfer cycles. Cases with retained products of conception were significantly more likely to have undergone vaginal delivery than cases without retained products of conception (78.0% vs. 61.1%); they were also more likely to have been complicated with the placenta accreta spectrum (24.2% vs. 0.45%). Among patients undergoing FETs, factors associated with a significantly increased risk of retained products of conception were embryo stage at transfer, use of hormone replacement cycles, and assisted hatching. Use of hormone replacement cycles represented the largest risk factor (adjusted odds ratio, 4.9; 95% confidence interval, 2.0-12.4), such that retained products of conception occurred in 0.05% (51 of 97,958) of deliveries after hormone replacement cycles but only 0.01% (5 of 47,079) of deliveries after natural cycles. Subgroup analysis showed that hormone replacement cycles and assisted hatching remained significant risk factors for retained products of conception in cases without polycystic ovary syndrome and anovulation and cases with vaginal delivery, but not cases with cesarean section. Among fresh embryo transfers, an increased number of retrieved oocytes was the only significant risk factor for retained products of conception.

Conclusion(s): Our analyses demonstrated that most of the cases involving retained products of conception were derived from FETs, and we identified the use of hormone replacement cycles as the largest risk factor for retained products of conception within this group.