With the exception of abstinence, no method of birth control is perfect. Half of all unintended pregnancies occur while a couple is using birth control. Why? The main reason is that whatever the method, it isn't being used correctly. If you are sexually active but not ready to start a family, here are some important things to know about birth control failure and how to prevent it.

Defining Effectiveness Rates

Woman holding home pregnancy test and looking worried — Sam Edwards / Getty Images

The effectiveness rates of birth control methods usually are described as either "typical user rates" or "perfect use rates." Perfect use rates refer to how effective a particular contraceptive or birth control method would be if it was used consistently and correctly a hundred percent of the time. Only a handful of methods have the potential to fall into the perfect use category. These include permanent sterilization (tubal ligation and vasectomy), Depo Provera, Nexplanon (birth control implant), and the IUD.

Typical use describes how the average person handles birth control, taking into consideration that it's subject to human error.

Types of Typical Use Errors

There are two types of typical use error when it comes to birth control:

Not Following Instructions: Condoms are a good example of how this can lead to birth control failure. It's important to make sure a condom fits correctly, for example, and to be careful when removing it. Whatever type of birth control you use, make certain you know how to use it.
Forgetting or Choosing Not to Use It: Let's say you're on birth control pills. It goes without saying, if you keep forgetting to take them or feel that it will be OK "just this once" to skip a pill, you're setting yourself up for unintended pregnancy. Create a fail-proof routine for popping your pills—store them next to your toothbrush so you're reminded every morning or switch to a contraceptive that you don't have to deal with on a daily basis, such as an IUD or implant.

Other Factors That Can Lead to Birth Control Failure

Besides human error, there are other potential reasons a particular method of birth control might fail. For example, certain medications ranging from antibiotics and antidepressants to diabetes drugs can make birth control pills less effective. Even some natural herbs and supplements can interfere. Hormone-based birth control may not work as effectively in women who are overweight or obese either.

What to Do If Your Birth Control Fails

If you suspect something went wrong with your birth control right away—for example, the condom broke—you can greatly lower the risk of becoming pregnant by using emergency contraception. This is a pill you can buy over the counter, no matter your age, to take within three to five days of a birth control accident or after having unprotected sex.

Of course, it's not always possible to know right away if your birth control has failed. The possibility will only come up if you or your partner's period is late. In that case, take a home pregnancy test right away. You'll have your answer sooner rather than later and will know what to do: Relax if it's negative, or see a doctor confirm if you really are pregnant.