The pill: the choice method of contraception for more than 11 million women in the United States.

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­If, 100 years ago, you told a woman that one day women would be able simply to swallow a tiny pill to avoid getting pregnant, she wouldn't have believed you. In 1909, condoms had already been in existence for hundreds of years, but depending on where you lived, they weren't always legal. Even if they were legal, they might not be culturally acceptable, affordable or reliable. Other than avoiding sex completely, the only other option a woman had might be the use of herbs or other folk methods. And those weren't always reliable, either.

The oral contraceptive pill, known simply as "the pill," is a marvel of modern chemistry. It's the most popular form of birth control in the United States. More than 11 million women reported using it in 2002 [source: CDC]. Today, it's legal, culturally acceptable, affordable and reliable. But it didn't come by all of those attributes so easily. The pill was first approved by the Food and Drug Administration (FDA) as a contraceptive in 1961, but until a 1965 Supreme Court ruling, only married woman in some states could obtain it. Until another Supreme Court ruling in 1972, unmarried women in some states still couldn't legally get it.

The pill is pretty straightforward on the surface. A woman takes one pill at the same time each day for 21 days. Then she either takes an inert pill (a placebo meant to keep her in the routine of taking a pill each day) or takes nothing for seven days while she has her period. Because the hormones in the pill keep her from ovulating, she shouldn't get pregnant.

The truth is, though, that some women who are on the pill do become pregnant. Some women experience side effects so unpleasant that they have to switch between different types of the pill or stop taking it altogether. And although it has been legal and available to all women since 1972, they might still have trouble obtaining it. Despite its popularity, the pill hasn't been without controversy throughout its history.

Let's start with looking at exactly how the pill works in a woman's body to prevent ovulation.