The human ovary contains a fixed number of primordial follicles that decreases bi-exponentially with age, culminating in the menopause at an average age of 50-51 years. There currently is no reliable test of ovarian reserve for individual women that will accurately predict their remaining reproductive lifespan. We use the Faddy-Gosden model of human primordial follicle population decline to describe the natural decay of the ovarian follicle pool. Assuming that the wide distribution for age at menopause is due to the wide variation in number of primordial follicles at birth, we describe follicle population decline for early and late menopausal women. Using published data on age-related ovarian volume as measured by transvaginal sonography, we have obtained a highly significant correlation between primordial follicle population and ovarian volume. We show that ovarian volume in women aged 25-51 years accurately reflects the number of primordial follicles remaining, and describe how measurement of ovarian volume by transvaginal sonography may determine ovarian reserve and reproductive age. The accurate assessment of ovarian reserve will revolutionize the management of women requesting assisted conception, those who have had treatment for childhood cancer and those who are considering delaying a family for personal or professional reasons.
