FDA to Decide Status of Morning-After Pill
Agency Nears Decision on Over-the-Counter Sale of Drug Known as 'Plan B'
WebMD News Archive
How Plan B Works
Women who do not want to get pregnant should use plan A first. The most effective way to avoid pregnancy is not to have sex. The next most effective way, other than permanent sterilization, is contraception. This means using barrier contraceptives such as diaphragms or condoms, IUDs, or various forms of hormonal contraception.
All these forms of contraception prevent a man's sperm from fertilizing a woman's egg. Depending on where a woman is in her menstrual cycle, emergency contraception may work the same way. But it is more likely to work after sperm fertilizes the egg.
Just as a seed isn't considered planted until it's put in fertile ground, the vast majority of doctors say a baby isn't conceived until a fertile human egg is implanted in a woman's womb. That happens about seven days after sperm and egg come together and the resulting pre-embryo travels down the fallopian tubes to the womb. Plan B keeps the fertile egg from getting planted in the womb.
If Plan B does not work -- if the egg does get planted in the womb -- the pregnancy will proceed normally, says David M. Plourd, MD, assistant professor of obstetrics and gynecology at Naval Medical Center, San Diego.
"It is an all-or-none phenomenon: With Plan B, you either don't get pregnant or you do," Plourd says. "There is no in-between. Ether it works or it doesn't. If it doesn't work, there are no effects on the pregnancy; no miscarriage and no birth defects."