How effective is the pill at preventing pregnancy?

The pill’s effectiveness depends on how consistently someone takes it. When used perfectly, the pill is more than 99% effective. But most of us aren’t perfect, so the pill’s effectiveness tends to be closer to 91%.

In other words:

Of those women who use the pill exactly as directed, fewer than 1 in 100 will experience an accidental pregnancy during the first year of using this method.

Of those women who do not use the pill exactly as directed, about 9 in 100 will experience an accidental pregnancy during the first year of using this method.