OBJECTIVE To determine if multinucleation in normally fertilized embryos is indicative of poor developmental or clinical pregnancy prognosis and to examine the ovulation induction characteristics associated with multinucleation. METHODS Retrospective review. METHODS A tertiary care institution. METHODS Patients undergoing IVF-ET cycles (exclusive of other assisted reproductive technologies). METHODS Cycles in which embryos had at least 1 multinucleated blastomere were compared with cycles in which all blastomeres exhibited no nucleus or a single nucleus (control). RESULTS When >50% of transferred embryos contained multinucleated blastomeres there was a significant reduction in implantation (3.4% vs. 14.7%), clinical pregnancy (9.1% vs. 29.1%), and live birth rates (7.5% vs. 27.6%) when compared with transfers of control embryos. In conjunction with this finding, multinucleate cycles had higher E2 levels and more follicles on the day of hCG administration, a higher number of oocytes retrieved, a higher fertilization rate, and more embryos transferred per patient than did the cycles that produced control embryos. When multinucleated embryos were present, but not transferred, the developmental capacity of the sibling embryo was reduced. CONCLUSIONS The evaluation of nuclear status using simple light microscopy is predictive of embryo developmental capacity and should be included in the embryo scoring system. The presence of multinucleated blastomeres in normally fertilized embryos is associated with a more effusive response to gonadotropin therapy and is indicative of a poor developmental outcome and lower clinical pregnancy rates.