Condoms are great at preventing both pregnancy and STDs. If you follow the instructions and use them every time you have vaginal, anal, or oral sex, there’s very little chance of pregnancy, or getting or giving an STD.
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.
The better you are about using condoms correctly every time you have sex, the better they’ll work. But there’s a small chance that you will get pregnant even if you always use them the right way.