Women ovulate only during the reproductive phase of their life. Nature predetermines a fixed number of oocytes while a woman is still unborn in her mother’s uterus and no new follicles are created during adult life. At birth she will have approximately 200,000 oocytes and will begin her reproductive cycles at puberty ovulating approximately 400 times during her reproductive years until menopause, which averages at age 51/52. Only a very small proportion of the oocytes ever make it to maturity, the vast majority do not make the grade or abandon their development along the way. Each month a small cohort of ovarian follicles, each containing an oocyte, begins to develop before a single dominant follicle takes over and becomes the one destined for ovulation in that cycle. The remaining follicles develop no further.
Ovarian reserve starts to decline after approximately 37 years old and is typically extremely low over the age of around 43. The lower the ovarian reserve, the less the ovarian response to fertility treatments, and the lower the quality of the eggs obtained. This is why IVF success rates are low in women over 41, despite the best of treatment methods.
Ovarian reserve assessment is generally advised for women seeking fertility treatment. It helps to understand an individual woman’s status with regard to her reproductive age. This is important when we wish to choose between mild therapies such as ovulation induction, or more involved treatments like IVF. While wishing to do the minimum to be effective, we are also conscious that unnecessary delay due to following inappropriate treatments may lead to further decrease in ovarian reserve.
It is not always easy to know the ovarian reserve exactly, every woman is different, but there are two tests that are commonly used either together or in isolation. The first of these is antral follicle count (AFC). Antral follicles represent a fraction of the ovarian reserve that has matured enough to be recruited for further development. The second is a blood test to measure Anti-Mullerian Hormone (AMH). AMH levels peak around the age of 25 and after a mild reduction remain stable until the late 30s, followed by a steep decline.
Whether it is for selecting the right fertility treatment, balancing ovarian stimulation requirements or freezing eggs to use later, an appreciation of ovarian reserve and ageing is informative and empowers women, enabling them to take control and make the right decisions regarding their fertility.
Around 6% of British women suffer unexplained early menopause, which is defined as menopausal symptoms before the age of 45. There is also some evidence that women with a family history of early menopause are more likely to experience it themselves – between 10 and 20% of women with a family history will experience it, compared to 7.4% of the general population.
As relevant ovarian reserve diminishes in the 15 years or so before menopause, it can be seen that women who suffer an early menopause are likely to suffer an equally early reduction in ovarian reserve.