Ovarian reserve is a term that is used to determine the capacity of the ovary to produce eggs that are capable of fertilisation resulting in a healthy and successful pregnancy. An assessment of ovarian reserve is of paramount importance in women who experience difficulty in conceiving as the result may highlight an otherwise unknown problem and influence the choice, type and timing of treatment in the future.
The ovary can be described as an egg bank from which the woman draws during her reproductive life. The human ovary contains a population of primordial follicles which can develop into mature eggs. At 18–22 weeks post-conception, the female ovary contains its peak number of follicles (up to 4 million). At birth the ovaries contain around only one million follicles and at puberty around 400,000 follicles. During the reproductive period of a woman’s life, one egg is released by ovulation each month and 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.
Previously the most commonly used test to assess this ovarian reserve was the 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. FSH levels are expected to be below 8 iu/l in women with good reproductive potential whilst levels of 8-12 iu/l are considered borderline. Levels of FSH above 12 iu/l suggest poor ovarian function. However, FSH levels may fluctuate widely from cycle to cycle and hence this is not a good test to predict ovarian performance in the future.
The latest scientific evidence suggests that the best currently available biochemical marker for ovarian reserve is Anti-Mullerian Hormone or AMH. AMH is produced by the antral follicles in the ovaries. Hence, as age increases and the number of follicles reduces, AMH levels decline. 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 the best way to assess ovarian reserve and future fertility.
Since AMH is secreted consistently throughout the menstrual cycle, testing for AMH can be performed at any time of the menstrual cycle.
A combination of an AMH test and a transvaginal ultrasound scan provides the best predictive test to determine future ovarian function.