What can I do to prevent pregnancy from happening after sex?

Use a condom. The best way to prevent a pregnancy is to use birth control or a condom before having sex. If you have unprotected sex you can take Plan B (the morning-after pill) within 3-5 days of unprotected sex. It is over-the-counter if you are 17 or older but you have to ask the pharmacist for it. A copper IUD can also be inserted up to 5 days later to prevent a pregnancy.