Immature oocyte retrieval: lessons from unstimulated IVF cycles

Objective: To describe retrieval of immature oocytes during unstimulated IVF and assess the in vitro maturation and fertilization rates. Design: Retrospective analysis. Setting: The USC program for assisted reproduction. Patient(s): Spontaneously ovulatory women with predominantly pelvic factor as their principal cause of infertility, under the age of 40, and no male factor. Intervention(s): HCG administration in mid-cycle, aspiration of all visible follicles, in vitro maturation of immature oocytes in culture media versus 50% follicular fluid in media, IVF, and embryo transfer. Main Outcome Measure(s): Rates of in vitro maturation, fertilization, and implantation after embryo transfer. Result(s): A total of 101 immature oocytes were obtained during 59 follicle aspirations. Thirty percent of prophase I oocytes matured to metaphase II in vitro compared with 44% of metaphase I oocytes. Fertilization rates for matured prophase I oocytes were 62% for those cultured in standard culture media (controls) and 87% for follicular fluid culture media. Two pregnancies resulted from the transfer of embryos derived from immature oocytes when no other embryos were transferred. Conclusion(s): Immature oocytes may be retrieved successfully during the mid-cycle aspiration of the dominant follicle in unstimulated IVF cycles. Maturation of immature oocytes in vitro with follicular fluid results in similar maturation and fertilization rates as for control incubation. Immature oocytes thus retrieved contribute to the overall pregnancy success of unstimulated IVF cycles. It may be better to retrieve immature oocytes during unstimulated cycles than during the follicular phase of natural cycles.