Then while she's growing up and getting older, the specified number is decreasing in quality and quantity. I hope the way I tried to simplify wasn't too idiotic because this is exactly how the things turn out in practice.
The decreasing quantity of egg-containing follicles in the ovaries is called "loss of ovarian reserve"
. Women begin to lose ovarian reserve before they become infertile and before they stop having regular periods.
Also a woman's age is the most accurate test of egg quality. An important change in egg quality is the frequency of genetic abnormalities called "aneuploidy" (too many or too few chromosomes in the egg).
Of course, you may lead healthy lives, have no bad habits, do whatever to improve your chances for getting pregnant but nothing really overrides the fact that age is doing its job steadily and efficiently. Namely, it's reducing our fertility.