When girls are born, they have between 500,000 and two million eggs in their ovaries, nestled in fluid-filled cavities called follicles. Sounds like a lot, but months earlier, before birth, they may have had as many as six or seven million eggs. By the time of puberty, only about 300,000 remain. Of these, only 300 to 400 will be ovulated during a woman's reproductive lifetime.

Fertility drops as a woman ages due to decreasing number and quality of the remaining eggs, but when your mother was born she was already carrying the egg that became you.