Those millions of immature eggs that were contained in your ovaries when you were young are contained in what are known as primordial follicles, and many of them die off as time passes. (Read more about the connection between age and fertility here.) When it came time for your first menstrual cycle, the first group of some of the surviving dormant follicles began to "wake up".
As they wake up, there is a sort of competition between them as they grow. Each month some of these primordial follicles stop growing, with only the best of the group moving onto the next month of growth. Of this group, only a select group will go onto becoming primary follicles, and then onto becoming what is known as secondary follicles. It's a bit like a follicle marathon to see who will get to become the winning egg.
Eventually, only one (and sometimes two) of these follicles become a mature egg to be ovulated.
But what causes these follicles to race in the first place?
