The probability of pregnancy in 100 episodes of sexual intercourse: A measure of male contraceptive performance.

Background: Accurately judging the efficacy of contraceptives is vital for preventing unintended pregnancy. The Pearl index and life table analysis describe female contraceptive performance. However, they are not ideal for quantifying male contraceptive efficacy given differences between male and female methods. In particular, male contraceptives like condoms are used "on demand" rather than long-term like female contraceptives. Additionally, the number of episodes of sexual intercourse, a key determinant of risk, is not considered. Lastly, men can father concurrent pregnancies in more than one woman. For these reasons, a male-specific measure may provide a more accurate measure of male contraceptive efficacy.

Objectives and methods: As each episode of heterosexual intercourse within a fertile couple is associated with an approximately 3% risk of pregnancy, the chance of an unintended pregnancy with a given number of episodes of sexual intercourse can be modeled with and without contraceptives of various effectiveness.

Results: Such modeling demonstrates that unintended pregnancy is strongly associated with both the number of episodes of sexual intercourse and the efficacy of the method. Based on these models, I propose a novel metric for male contraceptive efficacy called the Probability of Pregnancy₁₀₀, defined as the percent chance of an unintended pregnancy occurring with 100 episodes of intercourse. Probability of Pregnancy₁₀₀ should be easy for men to understand and is applicable to men with multiple sexual partners or men using "on-demand" contraceptives.

Discussion and conclusions: The prevention of unintended pregnancy by a male contraceptive is strongly influenced by both method efficacy and sexual frequency. Probability of Pregnancy₁₀₀ may offer a better measure of male contraceptive efficacy compared to the Pearl index and life table analysis as it takes sexual frequency into account; however, Probability of Pregnancy₁₀₀ will need to be tested prospectively in male contraceptive studies alongside the Pearl index and life table analysis to determine its utility compared to these existing measures of contraceptive efficacy.