By seven weeks' gestation, your fallopian tubes, ovaries, uterus and vagina were formed. Two months before you were born, your ovaries contained as many as five million eggs. That number dropped to about one million by the time you emerged from the womb, and to about 500,000 by the time you hit puberty. During your childbearing years, however, only about 450 eggs reach maturity and can be fertilized-still plenty for the 2.5 kids and white picket fence, if these were in your plan. The rest of the eggs slowly disintegrate. So by the time you hit menopause, only about 3,000 remain.

But the eggs are just one part of your reproductive system, all designed for the express purpose of creating, carrying and birthing a baby. Start with the external genital area. (See diagram below.) Called the vulva, this includes the labia minora, the inner lips enclosing the opening to the vagina; the labia majora, the outer, hair-bearing lips surrounding the opening to the vagina; and the urethra, the opening to the bladder.

The clitoris, a small, bud-shaped organ, is located just above the urethra-you can see and feel it if you try. Traveling through the muscular tube of the vagina, you reach the uterus, opening up like a room at the end of a long corridor. The opening of the uterus, the cervix, projects into the upper end of the vagina. You could feel it by inserting a finger into your vagina. The cervix also acts as a barrier to the uterus; it can't be penetrated by a penis, tampon or finger. Only when you're in labor does it (painfully) open.

The uterus itself is a hollow, muscular organ, about the size of a pear. Its lining I called the endometrium, which changes in thickness depending on where you are in your menstrual cycle. (If you get pregnant that menstrual cycle, endometrial tissue nourishes the embryo until the placenta grows.) Except during pregnancy, the cavity of the uterus is triangular and flat, its front and rear walls touching.

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Anatomy of the Uterus
The uterus has three layers. The outside layer is the peritoneum. This membrane secretes a blood-like fluid that partially covers the uterus. The middle layer of tissue, the myometrium, is very firm and muscular and makes up most of the uterus. The endometrium, a mucous membrane, is the innermost layer of tissue. It lines the uterine cavity and contains numerous glands, blood vessels and lymphatic spaces.⁴

On either side of the upper end of the uterus are the fallopian tubes. They're about four inches long and reach outward like fingers toward the ovaries. Tiny, hair-like projections on the ends of each tube catch the egg the ovary releases each month and funnels it along the tube down to the uterus. In addition to incubating eggs, the ovaries secrete the reproductive hormones estrogen and progesterone, which brings us to the next dimension of the reproductive system.

Honing in on Hormones
Hormones, the body's chemical messengers, determine when and how fast you grow, if you're hungry or satiated, whether you'll gain or lose weight; they even play a role in determining if you'll feel like making love tonight. They are part of a complex, related group of glands and tissues called the endocrine system.

In women, six key hormones play the leading roles in the reproductive system: gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, progesterone and testosterone (yes, women do have a bit of the male hormone, just as men's bodies secrete some estrogen).

Directing the continual hormonal play is a cherry-sized gland in the center of the brain called the hypothalamus-the ultimate link between body and brain. When it detects too much or too little of a particular hormone, the hypothalmus sends another hormone to its nearby partner, the pituitary gland, which, in turn, secretes its own hormones that instruct various other glands to turn up or down their hormone production.

Like a sentry, the hypothalamus keeps constant watch over the levels of both estrogen and progesterone, thus creating the mechanism that is supposed to keep these hormones in balance throughout your menstrual cycle.