A baby girl is born with all her eggs—1 to 2 million—already inside her ovaries. Men can usually produce sperm their entire lives, but for women it’s all a numbers game. Every month for about 25 years, our ovaries release a single mature egg, for a total of 300 during our reproductive years. Yet at birth our ovaries contain more than 1 million eggs, all our bodies will ever produce. The first day of our lives, immature eggs begin to self-destruct and are absorbed by the body, a process known as atresia. By puberty, a girl has about 300,000 eggs remaining. Our eggs are at their peak during our mid-to-late twenties. By our thirties, we may have plenty of eggs left, but the follicles that surround and release them become less responsive to hormonal signals. Why some women exhaust their egg supply at a young age, while others have no trouble getting pregnant and delivering healthy babies in their late thirties and forties is unknown. We each have a unique biological clock that’s set to a very individualized time.