How effective is birth control pills against pregnancy?

Answer 1

According to the CDC, the typical failure rate for Birth Control pills is about 9%, meaning that of 100 women who use that method of contraception for a year, 9 of them will get pregnant anyway.

The theoretical effectiveness is much higher (the failure rate is lower). The 9% figure takes into account that some people don't follow the instructions perfectly and do things like occasionally missing a dose for one reason or another. If you are very careful to never miss a dose and follow all the other directions, then the failure rate is closer to 1 pregnancy per thousand women per year.