Abstract / Summary
Conjoined oocytes are defined as two oocytes originating from the same follicle and have been identified in various mammals, including humans. Due to scarce data, their meiotic maturity, fertilization, and developmental potential remain poorly characterized, limiting evidence-based decisions in in vitro fertilization (IVF) laboratories. A retrospective cohort study was conducted in our center from January 2012 to September 2025. Thirty-eight patients (39 cycles) harbouring conjoined oocytes were identified. Clinical data of patients, oocyte morphometry, meiotic status, fertilization rate, and subsequent embryonic development were recorded and compared with matched normal oocytes from these patients. A total of 42 conjoined oocyte pairs were identified, displaying distinct zona pellucida (ZP) binding patterns. The overall maturation rate of conjoined oocytes was 36.90% (31/84), which was significantly lower than that of normal oocytes. Among these, 71.43% exhibited asynchronous maturation (MII-GV combinations), while the remaining cases included MII-MII, MII-MI, MI-GV, and GV-GV combinations. The larger oocyte in conjoined pairs had approximately twice the area of the smaller one; however, both were significantly smaller than normal oocytes. The ZP thickness of smaller oocytes was 12.05 ± 0.60 μm, which was markedly thinner than that of both larger and normal oocytes. Conjoined oocytes demonstrated a fertilization rate of 48.38% (15/31), significantly lower than those of normal controls. However, there were no significant differences in the proportion of high-quality embryos on day 3, usable blastocyst rate and high-quality blastocyst rate compared to controls. Preimplantation genetic testing for aneuploidy (PGT-A) testing revealed that 5 of the 8 blastocysts had normal chromosomal ploidy. Asynchronous maturation, thinner ZP, and reduced size collectively impair fertilization potential of conjoined oocytes. Nevertheless, once fertilized, cleavage kinetics and early embryo morphology are comparable to those of normal counterparts. These findings suggest, on a hypothesis-generating basis, that conjoined oocyte-derived blastocysts may be considered for transfer when no other viable embryos are available, pending future studies with pregnancy and live birth outcomes.