['To compare the implantation and pregnancy rates after cleavage stage embryo transfer (ET) with transfer of blastocyst-stage (days 5-6) embryos.', "Prospective randomized trial at an assisted reproduction unit in a university hospital. Women with six or more follicles at the last ultrasound scan before oocyte aspiration were randomized for transfer of a maximum of two embryos after 2-3 days (n = 80) or after 5-6 days (n = 64) of culture. Embryo quality, implantation and pregnancy rates were evaluated. Statistical significance was tested with the Chi-square test and Fisher's exact test.", 'No significant difference was observed in implantation rates (21.1% versus 20.9%, respectively) and clinical pregnancy rates (36.7% versus 32.5% respectively) after blastocyst and cleavage stage transfers for the two groups. The pregnancy rate among subjects who had at least one good quality embryo transferred was 37.5% per day 2-3 ET and 60% per day 5-6 ET.'] Is embryo transfer equally effective at cleavage stage and blastocyst stage : a randomized prospective study?