No form of contraception is 100% effective in preventing pregnancy. The “efficacy rate” of a contraceptive method describes approximately how effective that method is.
Efficacy rates measure what percentage of couples using a given method over the course of one year are statistically expected to conceive. For example, if a method is 99% effective, one in 100 couples who use it would be expected to conceive during the year. If a method is 80% effective, 20 out of 100 couples who use it would be expected to conceive during the year.
Efficacy rates are often divided into “perfect use” and “typical use.” Perfect use imagines that all couples always use the method exactly as intended. Typical use anticipates the usual rate of errors, such as forgetting to take a pill, inserting a diaphragm incorrectly, miscalculating fertile days, etc.
With a method such as the IUD, which requires no action on the part of the couple for years after insertion, there will be little or no difference between perfect and typical use.
Most of the efficacy rates cited on our website are based on this chart from the CDC (Centers for Disease Control and Prevention) in the US. Information on contraceptive efficacy is also available at the CDC and Planned Parenthood websites. Medical studies to determine efficacy can have conflicting results, so even very reliable sources may give different numbers. Sometimes, a given method may be more or less effective for a particular couple than the average rate. We recommend that each woman consult with her own physician about which methods will best meet her specific needs.
This article was updated on 21 March, 2022.