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On Aug. 24, 2006, the U.S. Food and Drug Administration approved a form of emergency contraception to be sold over the counter to women ages 18 and older. Above, a two-pill dose of the morning-after pill. See more drug pictures.

AP Photo/Findlay Kember

People can prevent unwanted pregnancies in many different ways. They can use condoms, oral contraceptives, injected contraceptives, IUDs, sponges and more. All of these methods can significantly reduce the chance of pregnancy. For example, condoms, when used properly, reduce the chances of pregnancy by 90 to 95 percent.

But if a couple has sex without using a contraceptive, or if the contraceptive fails (for example, a condom breaks), and the woman has no desire to become pregnant, what can she do?

On Aug. 24, 2006, the U.S. Food and Drug Administration approved a form of emergency contraception to be sold over the counter to women ages 18 and older. The generic name for this contraceptive is levonorgestrel. It's also known by the brand name Plan B.

The female reproductive system is extremely complex and the menstrual cycle involves several different hormones. Here's a simplified version of how it works:

  • Shortly after a woman finishes her period, her pituitary gland begins the monthly cycle by secreting FSH, or follicle stimulating hormone. This hormone tells the ovaries that it is time to prepare a follicle for ovulation.
  • One follicle develops and begins emitting the hormone estrogen. Estrogen causes the uterine lining to thicken so that it will be ready to accept a fertilized egg.
  • Just before ovulation, the ovaries also secrete progesterone and continue secreting it for about two weeks.
  • The hypothalamus and pituitary gland sense the level of estrogen rising in the blood. When the level is high enough, the pituitary gland releases luteinizing hormone (LH). This hormone tells the follicle to release the egg into the fallopian tube. Sperm can fertilize this newly released egg during a 24-hour window.
  • If sperm fertilizes the egg during that 24-hour window, and if conditions are right, the fertilized egg implants itself into the thickened uterine lining and the woman is pregnant. If the egg goes unfertilized or if something prevents implantation, the woman doesn't get pregnant and she begins menstruating about two weeks after ovulation. Then the cycle repeats.

Although there is only a 24-hour window during which an egg can be fertilized, sperm can live for three to five days inside a woman's body. So if a woman has unprotected sex three days prior to ovulation, she has a very good chance of getting pregnant. (See How Sex Works for more information.)

On the next page, learn how the morning-after pill could lower the risk of pregnancy.