A condom is a sheath-shaped barrier device, used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection. If you use condoms perfectly every single time you have sex, they're 98% effective at preventing pregnancy. But people aren't perfect, so in real life condoms are about 85% effective — that means about 15 out of 100 people who use condoms as their only birth control method will get pregnant each year.