The purpose of stimulation of the ovaries is to bring several oocytes to maturity at the same time. This is necessary because not every oocyte is ready to be fertilized and not every embryo will implant in the uterus.

Hormonal stimulation is monitored very closely by ultrasound examinations and determinations of hormone levels in the blood. Controls are run to check the number and size of follicles and the thickness of the uterine mucosa. In this way the most favourable time for oocyte recovery can be ascertained.