Recent improvements in cell culture systems have led to a shift in in vitro fertilization (IVF) practice from early cleavage stage embryo transfer (ET) to blastocyst transfer. Blastocyst culture and transfer could theoretically provide better synchronization between the embryo and the uterine endometrium. The purpose of this study is comparing pregnancy rates and implantation rate following transfer of thawed cleavage embryos and blastocysts cultured from thawed cleavage embryos.