Sarah is 34. She is a stockbroker. She has worked hard to establish her career and consequently, she has had little time for men. Her friends have pointed out that her “biological clock” is ticking. She read an article about the possibility of social fertility preservation. She comes to see you and asks if you can tell her how long she will be fertile for? She also asks if she should consider social fertility preservation. The essential cause of the existence of the menopause is that the ovary contains a finite number of follicles: these are progressively lost with time until insufficient remain to support menstrual cyclicity. Ovarian follicles are both the source of the female gamete, and the key site of reproductive hormone production. Depletion of follicle numbers therefore results in both loss of fertility and gonadal estrogen production, and thus differs substantially from the situation in the male where the two functions of the gonad are anatomically and functionally more independent, and loss of one does not necessitate loss of the other. Establishment and loss of the ovarian reserve Ovarian follicles are formed during fetal life, with primordial follicles first seen in the ovary from about 18 weeks of gestation. Prior to this, the female germ cells have been specified, migrated to the gonadal ridge where they have proliferated, before they exit mitosis and enter meiosis only to arrest at diplotene of meiosis 1. During this process they reorganize their interactions with surrounding somatic cells to form primordial follicles. This process is complete by birth and indeed some newly formed follicles start growing immediately so that during later fetal life and throughout childhood the ovary contains follicles at a range of stages of development, although the later development of antral follicles to ovulatory stages does not, of course, occur until after puberty when there is sufficient gonadotropic stimulation to support them through to completion of growth and maturation.