Ovarian reserve is a term that is used to determine the capacity of the ovary to provide eggs that are capable of fertilisation resulting in a healthy and successful pregnancy. While there is no known method for assessing the ovarian reserve of individual women, indirect determination of ovarian reserve is important in the treatment of infertility/ subfertility.
The ovary is generally thought of as an egg bank from which the woman draws during her reproductive life. The human ovary contains a population of primordial follicles. At 18–22 weeks post-conception, the female ovary contains its peak number of follicles (about 400,000 in the average case, but individual peak populations range from 35,000 to 2.5 million). While each month one egg is released by ovulation the remaining follicles that were recruited towards maturation are lost by atresia. Few if any oocytes are replenished during the reproductive years. Thus with advanced maternal age the number of eggs that can be successfully recruited for a possible pregnancy declines. Attempts have been made to assess the number of potential useful oocytes in a noninvasive way.
Previously the most commonly used test to assess this ovarian reserve is the day 1-4 FSH test. This blood test determines the level of FSH between cycle day 1 – 4. This time of the menstrual cycle is chosen because at this time the oestrogen level is expected to be low, a critical feature, as FSH levels are subject to a negative feedback. Thus any determination of FSH needs to include the corresponding oestradiol level to indicate that the FSH level was drawn, when the estrogen level was low. In a patient with infrequent menstruation, an FSH level and estrogen level could be measured at random and is valid if the estrogen level is low. Generally FSH levels are expected to be below 8 iu/l in women with reproductive potential (levels of 8-12 iu/l are considered borderline). Although FSH and more recently Inhibin B have been shown to have some correlation with ovarian reserve, it is now well established that Anti-Mullerian Hormone or AMH is a more useful biochemical test. High levels however can be present in women with Polycystic Ovarian Syndrome which compromises female fertility and therefore a combination of AMH and a transvaginal ultrasound to count the number of antral follicles is probably the best way to assess ovarian reserve and future fertility. This combination is sometimes referred to as the Biological Body Clock Test.
Another approach is to examine the ovaries by transvaginal ultrasound and to determine their size as ovaries depleted of eggs tend to be smaller and to examine the number of antral follicles visible.