Oocyte versus embryo vitrification for delayed embryo transfer: an observational study.

The aim of this observational prospective study was to compare multiple embryological and pregnancy outcomes for vitrified oocytes against the same outcomes using vitrified embryos in patients at risk for ovarian hyperstimulation syndrome. Ninety-six patients were included and allocated to vitrification of oocytes (Group 1) or embryos (Group 2). No statistical differences in baseline characteristics between groups were detected. Implantation rate was 30.6% versus 33.1%, and clinical pregnancy rate was 41.9% versus 7.1% in groups 1 and 2, respectively. A higher clinical spontaneous abortion rate occurred in group 2 (9.7% versus 21.9% for groups 1 and 2, respectively), but the same cumulative clinical pregnancy rate was observed after three embryo transfers (62.0% in group 1 and 69.6% in group 2). The ongoing pregnancy rate per patient was similar in both groups (56.0% and 54.3% in groups 1 and 2, respectively). Also, live birth rate per stimulation was similar (72.0% and 69.6% in groups 1 and 2, respectively). No differences were observed in outcomes according to vitrification timing. Oocyte vitrification achieved the same live birth rate as embryo vitrification.