Sex Ratio of Live Births Following Fresh Versus Cryopreserved-Thawed Embryo Transfer in Invitro Fertilization: A Retrospective Study

<b>Background:</b> Human sex is genetically determined at fertilization; however, emerging evidence suggests that assisted reproductive technologies (ART) may influence the secondary sex ratio (SR). Factors related to parental characteristics, laboratory practices, and embryo handling have been implicated, though findings remain inconsistent. This study aimed to compare the secondary sex ratio of live births following fresh versus frozen–thawed single blastocyst transfer.<br /> <b>Methods:</b> We conducted a multicenter retrospective cohort study reviewing IVF records from Kingswill Specialist Hospital, Lagos, and referrals to the 68 Nigerian Army Reference Hospital and Lagos University Teaching Hospital. Records from 1 January 2017 to 31 December 2025 were assessed. Women who achieved live birth following single blastocyst transfer in either fresh or frozen–thawed IVF cycles were included.<br /> <b>Results:</b> Of 159 records screened, 144 met eligibility criteria and were analyzed (72 fresh and 72 frozen–thawed cycles). Fresh embryo transfer was associated with a higher proportion of male live births compared with frozen–thawed transfer (69.4% vs 45.8%), whereas frozen–thawed cycles yielded a higher proportion of female infants (54.2%).<br /> <b>Conclusion:</b> Fresh single blastocyst embryo transfer was associated with a higher male-to-female live-birth ratio compared with frozen–thawed blastocyst transfer, despite transfer at an equivalent developmental stage. These findings suggest that embryo cryopreservation may be associated with differences in sex ratio following IVF.