Impact of oocyte cryopreservation on embryo development.

Objective: To verify whether the morphologic evaluation of zygotes and embryos derived from thawed oocytes could provide some relevant information regarding their developmental performance.

Design: Fertilization, zygote, and embryo morphology from sibling fresh and frozen oocytes was compared.

Setting: Reproductive Medicine Unit, Società Italiana Studi Medicina della Riproduzione, Bologna, Italy.

Patient(s): Two hundred thirty-four patients underwent intracytoplasmic sperm injection cycles from which 1,101 spare metaphase II oocytes were cryopreserved. Subsequently, 256 thawing cycles were performed, and 997 oocytes were thawed.

Intervention(s): Intracytoplasmic sperm injection was performed on both fresh and frozen oocytes.

Main Outcome Measure(s): Fertilization rates, pronuclear zygote morphology, and embryo cleavage rates.

Result(s): Thawed oocytes had lower chances of being fertilized and developing into top-quality zygotes and regularly cleaving embryos when compared with sibling fresh oocytes irrespective of female age. As a result, the percentage of transferred cycles was significantly lower in frozen cycles compared with fresh cycles (79% and 93%, respectively); the proportion of transferred top-quality embryos followed the same trend.

Conclusion(s): Reduced fertilization and cleavage rates in frozen cycles when compared with sibling fresh oocytes suggest that, even if surviving thawing, the process of slow freezing has a negative impact on the potential of further growth that is evident as early as the first cleavage divisions.