Misuse and failure of birth control are major contributors to the millions of unplanned pregnancies in the United States each year. When failure rates of contraceptives are mentioned, they usually refer to a given year of use. Less understood is that the risk of failure is compounded over time. The longer any method of contraception is used, the greater the probability of unplanned pregnancy — the same way that any small risk, taken repeatedly, grows in likelihood. This is true for all contraception methods, even in the highly unlikely event that they are used perfectly, every time.

These charts show probabilities of pregnancy while using each method, alone, for up to 10 years, by two standards: Typical use: This is the norm, reflecting the effectiveness of each method for the average couple who do not always use it correctly or consistently. (The methods are shown in order from least to most effective, under typical use.) Perfect use: A measure of the technical effectiveness of each method, but only when used exactly as specified and consistently followed. Few couples, if any, achieve flawless contraceptive use, especially over long time periods.