Theoretically, both the combined (estrogen/progestin) and mini (progestin only) birth control pills are over 99% effective - if used correctly. That's a very big "if," however. In practice, most women make mistakes occasionally and so the actual effectiveness rate is only around 95%. There are two common types of user error that can lead to pregnancy.

Starting Late

The most common error women make using the pill is starting their next cycle one or more days late. If you start your next cycle one day late, take two pills immediately and then one pill per day after that. If you procrastinate even more and start two days late, take two pills for each of the next two days and one pill per day after that. If you really drop the ball and are three days or more late starting your next cycle, it's probably too late for this month. You should immediately switch to another form of temporary birth control and call your doctor for instructions. It's possible you'll be fine depending on the particular pill you're on, but better safe than sorry.

Skipping A Day

Another common error women make while on birth control pills is forgetting and accidently skipping a day. If you miss one day sometime during your cycle, immediately take the pill you missed and continue with the rest of your cycle as planned. This may mean you have to take more than one pill in a day. If you miss two pills in a row any time during your cycle, take two pills the day you remember and two the next, continuing with the rest of your cycle as normal. If this happens, however, you should definitely use a backup form of birth control just to be on the safe side, because you are at risk.

Mistakes while using the pill are not uncommon. The first thing you should do is switch to a backup form of birth control or refrain from sexual intercourse entirely until you're caught up. Of course, only your doctor can advise you on whether or not it's safe for you to take multiple pills in any given day, as there may be adverse hormonal side effects.