As far as oxygen (O2) tension is concerned, both oocytes and embryos from different mammalian species included the human are normally surrounded by values which are much less than half of atmospheric O2, ranging from high (8.7% O2) in the rabbit oviduct, rabbit and hamster uterus, to as low as 11 mm Hg (1.5% O2) in the monkey uterus. Even in the preovulatory follicle, oocytes are bathed within follicular fluids containing oxygen that range from > 1 to 5.5 % The aim of this study was to evaluate the impact the effect of low oxygen tension after day-3 embryo transfer in our egg donation programme on pregnancy and implantation rates.