Even the most accurate forms of birth control will not prevent pregnancy 100% of the time.
Condoms for example can break due to many causes such as high heat, the type of lubricant used, or even its shelf life. With perfect use, you still have a chance of getting pregnant.
“The Pill” is another common form of birth control. This has a 5% failure rate. Pregnancy can occur if it is not taken correctly. Examples include taking the pills too late in your menstrual cycle; two or more pills missed in a row; not taking them in the correct order; even half a day late taking a lower dose pill; taking them while on antibiotics; being a heavy smoker; or being overweight. These can all increase your chances of getting pregnant.
There are many other types of birth control that you may have used. It is important to understand that even when used correctly, there is still a chance of getting pregnant.