Howstuffworks "How does the morning-after pill work?"

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 August 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 or older. The generic name for this contraceptive is levonorgestrel. It's also known by the brand name Plan B.

AP Photo/Findlay Kember
A two-pill dose of the morning-after pill

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.

Image courtesy Georgia Reproductive Specialists
A fertilized egg
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 does not 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.

You can see from this description that a drug could lower the risk of pregnancy in one of three ways:

What Plan B Isn't Levonorgestrel is an emergency, or back-up, contraceptive. It can help reduce the chance of pregnancy if the woman is not already pregnant. Levonorgestrel has no effect on the mother or the fetus if the woman is already pregnant. RU-486, or mifepristone, is a completely different drug that performs a chemical abortion early in a pregnancy.

It could kill all of the sperm after ejaculation.
It could prevent the fertilized egg from implanting in the uterus.
It could either prevent or delay the release of the egg. Levonorgestrel takes this third path.

When you purchase Plan B, you get two pills. Each pill contains 0.75 milligrams of levonorgestrel. You take the first pill as soon as possible after unprotected sex, and you take the second pill 12 hours later.

Although scientists aren't completely sure how it works, they believe that levonorgestrel prevents pregnancy either by stopping the ovulation process or by disrupting the ability of sperm and egg to meet in the Fallopian tubes. Some speculate that the drug may prevent the fertilized egg from implanting as well, perhaps by making the uterine lining less receptive to the egg.

Levonorgestrel does this by disrupting the natural hormonal cycle. It contains a synthetic form of progesterone (regular birth-control pills contain it in lower doses). The high doses of progesterone in Plan B are disruptive enough to prevent fertilization or implantation.

If ovulation has already occurred, levonorgestrel will be less effective. It will be most effective if it is taken before ovulation. This is why it is important to take Plan B as soon as possible after unprotected sex. Once a fertilized egg implants, Plan B will have no effect. This explains why doctors advise women to take Plan B no later than 72 hours after unprotected sex – the chances of it working are very low at that point.

Because of all these variables, Plan B is not 100-percent effective. But in clinical trials, it has been found to be 89-percent effective.

For more information about contraception, pregnancy and more, check out these links: