Mathematics of Birth Control Effectiveness

[firegirl81]

Hi Scarleteen!

When you discuss the effectiveness of various birth control methods (i.e. in your article "Birth Control Bingo") what exactly do the percentages mean? In other words, if a method is said to be 97% effective, does that mean that if a couple has sex 100 times, they will on average get pregnant 3 of those times? Or does it mean that if 100 couples have sex regularly for a year, on average 3 of them will become pregnant? I've heard birth control effectiveness percentages explained both ways, and I'm wondering which your information is based on.

Thanks!

[Karyn]

It's the second one: for every 100 couples using a method that is 97% effective over the span of one year, 3 pregnancies would be expected. That's how effectiveness is measured in any clinical trial of any method, and how the CDC, for example, defines effectiveness.