Assessment of fertilization and embryo evaluation Fertilization was assessed as the number of 2PN zygotes per COC retrieved. Embryo evaluation was performed on days 2 and 3 by recording number of blastomeres and percentage of fragmentation. Grade A embryos were defined as embryos without anucleate fragments. Grade B embryos had blastomeres of equal or unequal sizes, with a maximum of 20 % of the volume of the embryo filled with anucleate fragments. In grade C embryos, anucleate fragments were present in 20-50 % of the volume of the embryo. Grade D embryos had more than 50% of the volume of the embryo filled with anucleate fragments. The embryos were evaluated daily until day 5. On day 5, the embryos were classified into arrested embryos showing no signs of compaction on day 5, compacting embryos (C1-C2), early blastocysts (Bl1-Bl2) and expanding blastocysts (Bl3-Bl7) according to the classification proposed by Gardner and Schoolcraft (1999). Embryos were considered for transfer at day 5 only from the compacting stage on. In both groups, supernumerary embryos were cryopreserved on day 5 or 6.