This study will determine whether the use of low oxygen tension during "in vitro" embryo culture may increase the pregnancy rates in women undergoing egg donation.

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.

Inclusion Criteria:

• Women aged 25-45 years of age;
• In good mental and physical health;
• First or second of egg-recipient treatment;
• Embryo transfer on Day 3.
• One to three embryos will be transferred.
• BMI between 18-25 kg/m2.

Exclusion Criteria:

• Uterine pathology or hydrosalpinx
• Recurrent miscarriage
• No chronic illnesses