When a girl is born, it is estimated that her ovaries contain between 400.000 and one million egg cells. From that moment on, the number of egg cells starts to decrease. The egg cells contained in the ovaries are what doctors call the “ovarian reserve”. This ovarian reserve gets smaller over time. In general, a woman’s fertility is closely related to the ovarian reserve, in a way that the bigger the ovarian reserve, the higher the fertility.
From 35 years onwards, the decline of the ovarian reserve is sharper. It is estimated that at 40 years old, only 20% of women has a normal ovarian reserve. These are women with a potentially normal fertility. Instead, 10% of women at 40 will have already experienced menopause (early onset). When reaching menopause, the ovarian reserve has been completely exhausted.
So, the ovaries do not produce any more egg cells than it already contains when the girl is born. So far, there has not been found a way to avoid the ovarian reserve getting exhausted over time. As a result, a woman’s fertility is directly related to her age, and declines significantly after 35 years of age. Nor can we predict when a specific woman’s ovarian reserve will be exhausted (or the other way around, we do not know until when a woman conserves her fertility).