You've been coping with it every month since you were a young girl, but just how much do you know about your menstrual cycle? No, not the four or five days of bleeding you get every month, but the full 28-day cycle during which your hormones rise and fall with greater regularity than the stock market. That's what we thought. Welcome, then, to Menstrual Cycle 101.
In the Beginning
Talk about the family jewels! Did you know that when you were born you had more than 2 million follicles, or "pre-egg" cells, in your minuscule ovaries? By the time you reach the age of menstruation, that number has shrunk to 400,000; still pretty remarkable. Obviously, though, you're not going to have a half-million babies. So why so many egg follicles? We really don't know. Think of it as nature's way of providing redundant systems.
In terms of absolute numbers, between 300 and 500 of those follicles will ripen into eggs by the time you reach menopause. The more pregnancies you have and the longer you breastfeed (both of which suppress ovulation) the fewer eggs you'll release. And, just like the rest of your body, those follicles age along with you; so by the time you reach your 30s and 40s—an increasingly popular time to have a first child—they're not as, ahem, fresh as they were in your early 20s, so it may take longer to get pregnant.
Of Glands and Hormones
Back to the menstrual cycle. Around age 11 or so (it seems to get earlier with every generation), and every month until menopause, a region at the base of the brain called the hypothalamus revs up production of a chemical called gonadotropin-releasing hormone follicle (GnRH). This hormone does just what its name implies. It sends a message to the pituitary gland (also in the brain) to release follicle stimulating hormone (FSH) and, while it's at it, a bit of luteinizing hormone (LH).
Together, the two hormones—FSH and LH—tell your ovarian follicles to start ripening into an egg.
It takes about 14 days for the follicles to fully ripen. As they mature, they release estrogen, increasing blood levels of this ultimate feminine hormone. The estrogen has its own job to do—signal the uterine lining to thicken.