It may be surprising to learn that, by the time girls are born, they already have lost a very large part of their inherent ovarian function. As we now have come to understand, ovarian function directly appears to relate to the number of eggs (also called oocytes) that are left within the ovaries. In other words, the more eggs are left within a woman’s ovaries, the better her ovarian function.

Surprisingly, the largest number of eggs is found in females while they still are fetuses, safely hidden away in their mothers’ uteri, at approximately 16 to 20 weeks of pregnancy. At that time, eggs become encapsulated by other ovarian cells, forming tiny cystic spaces, called follicles (at this stage, a so-called primordial follicles). By the time a female is born, the number of eggs/follicles in her ovaries has spontaneously reduced from a peak of 5 to 6 million, to approximately 1 to 2 million. In other words, at birth a female has already lost approximately 80% of her initial egg volume and, with it, approximately 80% of her original, theoretical ability to reproduce.

The process of egg loss continues, however, unabated: By the time puberty is reached, only approximately 300,000to 500,000 eggs remain within the ovaries, and this number continues to decrease at a steady pace until full menopause is reached, for most women at an approximate age of 51 to 52 years. It is generally believed that at that point only approximately 1,000 oocytes remain within the ovaries. Menopause is reached when menstruation has stopped permanently for a one-year period.

On the way towards menopause a number of key points in ovarian senescence deserve a little more attention: We already mentioned puberty, when a female starts menstruating and establishes the process of ovulation (release of an egg which is ready to be fertilized by a sperm) and, therefore, commences the reproductive phase of her life. Egg loss, and with it loss of fertility potential, continues from that point on unabated, though at a rather moderate pace until approximately age 37 to 38 years. At that point, approximately 25,000 eggs are believed to remain within the ovaries and loss of fertility accelerates to a much quicker pace from there on. This is believed to be the principle reason why it is so much more difficult to achieve pregnancy after age 38, and especially after age 40.