The human female reproductive system is tightly regulated to make sure that one healthy egg is produced each month. Interactions between the ovaries and the brain regulate the development of the eggs and control their release during ovulation. If this regulation is dusrupted, women may experience infertility. This article explains how knowledge of the menstrual cycle is used clinically, either to stop ovulation (which reduces fertility) or to produce a lot of eggs for in vitro fertilisation.