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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. Why? Well, remember that all these hormones and hormonal responses are designed with one goal in mind: to prepare your body for a baby. The thicker uterine lining provides nutrients for a fertilized egg.
Once estrogen levels reach a certain point, the hypothalamus kicks into gear again, releasing more GnRH which stimulates the pituitary to release more LH. A lot actually. Think of it as the "surge" of the menstrual cycle. The surge has one goal: to get one of those ripening follicles to go all the way and pop out an egg. This surge typically starts around day 12 of your cycle and lasts about two days.
Congratulations. You have now ovulated. Before you can say "condom," the egg is sucked into the fallopian tube where it makes its way to the uterus over several days. If you want to get pregnant (or even if you don't want to) this is when it happens. Thank that high estrogen level; it thinned your cervical mucus, opening the gates, as it were, for sperm to swim past the cervix into the fallopian tubes to reach the egg.
This entire process—from the first spurt of GnRH to the popping out of that egg—takes about two weeks, by which point you're in the middle of your cycle. This is when you might find yourself reaching for your partner more often because high levels of estrogen pique your libido. Why? Recall the ultimate goal: pregnancy.
This is also where birth control pills come in. They suppress ovulation by maintaining steady levels of estrogen and progesterone. Without that surge in estrogen, your ovaries don't get the signal to mature an egg, so you don't ovulate. No ovulation, no pregnancy.
You're now in what's known as the "luteal phase" of your cycle. So let's go back to that follicle. It's done its job and changed its color (to yellow) and its name (to the corpus luteum). It still has one more assignment, however: to release more estrogen and lots of progesterone. Progesterone keeps that uterine lining in place in the hope that you get pregnant. It also increases your body temperature, which is why women trying to get pregnant take their temperature to track their cycle.
If you do get pregnant, the fertilized egg makes its way to the uterus and implants in the lining, where it releases yet another hormone to keep the corpus luteum producing progesterone. If you don't get pregnant, the corpus luteum gives up, turns white and shuts down the progesterone faucet, in turn signaling the uterine lining, or endometrium, to also give up and let go. In other words, your period starts.
One more thing: Since this is a cycle, those falling progesterone levels also have another effect; they signal the hypothalamus to release GnRH, starting the whole thing over again.