Now that we understand that ovarian function is closely related to the number of eggs/follicles left within the ovaries at any given point, the obvious question that needs to be asked is, of course, where do all these disappearing eggs go? The answer to this question is quite complicated since there are a number of different mechanisms by which egg/follicle numbers get depleted over a lifetime. As we noted above, the largest loss in primordial eggs occurs during the second half of pregnancy. No other phase, during life, will even approximate the extent of loss the female fetus incurs during that time period. Most of this loss takes place through a process, called atresia, which results in the degeneration of eggs and their removal by scavenger processes.

Once a female enters her reproductive lifespan and starts ovulatory cycles, The ovaries initiate a monthly recruitment process. What this means is that every month, between 500 and approximately 1,000 eggs/follicles are selected from the remaining oocyte pool and start a maturation process, which will take them over an approximately 4-months period from the most immature follicular stages (the so-called primordial follicle), through the primary follicle, the preantral follicle, the antral follicle, the preovulatory follicle into ovulation. In a natural cycle only one follicle usually ovulates to release an egg. Consider the loss factor: out of 500-1,000 follicle that started the journey through maturation, only one, finally, reaches ovulation. No wonder, the number of eggs within the ovary continues to decline quite rapidly.

Again, various scavenger processes remove a vast majority of the initially recruited follicles. The most prominent is a process, called apoptosis.

As women age, the amount of eggs/follicles, recruited every month, declines. In younger years this number will be closer to 1,000. As the female advances into her 30-ies, the number increasingly approaches 500. Together with this decline in numbers, the quality of eggs also decreases. One of the main reasons for this decline in quality appears to lie in the fact that the genetic material within the eggs ages, and, as a consequence, more chromosomal (genetic) abnormalities occur in eggs and embryos. This is reflected in greater difficulties getting pregnant with advancing age, higher miscarriage rates and higher rates of chromosomal abnormalities in ongoing pregnancies. As a consequence, women who conceive after age 34 to 35 are, therefore, usually advised to undergo prenatal genetic testing of their pregnancies.