How it Works
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.
You can see from this description that a drug could lower the risk of pregnancy in one of three ways:
* 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.
What Plan B Isn’t
Levonorgestrel is an emergency, or backup, contraceptive. It can help reduce the chance of pregnancy if the woman isn’t 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.
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’s taken before ovulation. This is why it’s 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.