Pregnancy and birth resulting from transfer of a blastocyst observed to have one pronucleus at the time of examination for fertilization.

This case report describes a successful full-term pregnancy and birth after the transfer of a zona-free blastocyst derived from an oocyte observed at fertilization check as having only one distinct pronucleus (PN). The patient had previously undergone four in-vitro fertilization (IVF) cycles and three frozen embryo transfer cycles, with one pregnancy resulting. In this IVF cycle, 7/19 oocytes were fertilized exhibiting two distinct PN; however, all these oocyctes failed to develop in culture and had arrested or totally fragmented by day 6 after insemination. One oocyte (1/19) displayed only one PN 18 h after insemination, but culture of this oocyte led to development of an early cavitating blastocyst by day 6. Since no other embryos were available for transfer to the patient, this embryo was transferred, resulting in a full-term pregnancy with delivery of a normal healthy boy. Observation of a single PN at the normal time of fertilization assessment would not appear to be an absolute indicator of developmental incompetence. In-vitro culture to 6 days post-insemination provides the opportunity to assess embryological development after activation of the embryonic genome. Formation of a morphologically normal blastocyst may be an indicator of a fertilized embryo with normal developmental capacity.