Incidence of Retained Products of Conception Following Hormone Replacement Versus Natural Cycles in Frozen Embryo Transfer.

Purpose: This study examined whether the incidence of retained products of conception (RPOC) differs between hormone replacement cycles (HRC) and natural ovulation cycles (NC) in frozen-thawed embryo transfer (FET) pregnancies. Methods: We retrospectively analyzed 425 clinical pregnancies following FET at a single center from 2017 to 2021. Pregnancies were classified as HRC (n = 204) or NC (n = 221). The primary outcome was RPOC incidence, diagnosed by ultrasound, Doppler, and histopathology when available. Management approaches and perinatal outcomes were descriptively assessed. Exact tests and Firth's penalized logistic regression were applied given the small number of events. Results: RPOC occurred in 14/425 pregnancies (3.3%), with 11/204 (5.4%) in HRC and 3/221 (1.4%) in NC. The Cochran-Mantel-Haenszel odds ratio was 4.47 (95% confidence interval [CI] 1.29-16.39), and the adjusted Firth odds ratio was 3.92 (95% CI 1.18-12.96). Among RPOC cases, 5 were managed conservatively, 8 required uterine evacuation, and 1 required uterine artery embolization; histopathological confirmation was obtained in 7 cases. No significant associations were observed with embryo-transfer-related procedures. Conclusion: HRC was associated with a higher RPOC incidence than NC. These exploratory findings suggest potential protocol-related differences in placental separation and warrant validation in larger, prospective multicenter studies.