While there are a ton of very good health reasons to go on birth control, many people seek out oral contraceptive pills to prevent getting pregnant. And broadly speaking, the pill can do that pretty well: the pill emits hormones that prevent ovulation and thicken the cervical mucus so it's harder for sperm to enter into the uterus and find an egg. But unfortunately, hormonal birth control pills are not completely foolproof, and it is still possible to get pregnant on the pill.
Birth control failure rates are usually calculated based on "typical use," which reflects human error, like the fact that people may not take their pill at the same time everyday. "Perfect use," on the other hand, refers to how the drug works when it's taken exactly according to instructions. However, your own likelihood of getting pregnant on the pill largely depends on which type of birth control pill you're on, and whether or not you remember to take the pill at the same time everyday.
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In general, a combination pill, which contains synthetic estrogen and progesterone, tends to be more effective at preventing pregnancy than a progesterone-only pill. In fact, progesterone-only pills are often referred to as "mini pills," because they contain a lower dose of hormones than a combination pill, and because they're not as good at inhibiting ovulation. For this reason, as many as 13 out of 100 women will get pregnant on a progesterone-only pill in a year, while nine out of 100 women on a combination pill will get pregnant within a year, according to the Mayo Clinic.
If your only form of birth control is the pill, then these stats may freak you out a little bit. In order to make your birth control as effective as possible, you've got to take your pill every single day at the same time — and that's especially true if you're on a progesterone-only pill. Birth control pills aren't a fit for everyone, so if you find that sticking to a rigid schedule just isn't feasible for your schedule, then you may want to consider a different type of birth control, like an intrauterine device (IUD) or other long-acting reversible contraceptive. For comparison's sake, a hormonal IUD has a failure rate of 0.2%, and a copper IUD has a 0.8% failure rate, so you might want to consider going that route.
At the end of the day, preventing pregnancy entails more than just remembering to take your pill. Another way that you can prevent pregnancy is by using a backup birth control method, like condoms or spermicide, or avoiding penis-to-vagina sex altogether on days when you know you're most fertile. While that might seem like a lot more work than you're down for, if you're trying to prevent pregnancy, it's worth it for that 9-13% chance that you end up with a baby.