All women included will undergo artificial ovarian stimulation with GnRH antagonist down-regulation with daily injections of either ganirelix (Orgalutran®) or cetrorelix (Cetrotide®). Treating physicians will opt on which exogenous gonadotropins should be used according to the patient's profile and preference and can include either recombinant follicle stimulating hormone (FSH) or highly purified urinary human menopausal gonadotropin (HP-HMG). Ovarian stimulation will commence after it is confirmed that the patient is not pregnant and has basal levels of oestradiol, progesterone, FSH and luteinising hormone (LH). The stimulation will be monitored simultaneously by pelvic ultrasound and hormonal analyses (oestradiol, progesterone), starting on day 7 of stimulation and then every 1 to 3 days, according to the individual endocrine profile and follicular development.