Q: Can I have sex without protection while my girlfriend is on her period and she won’t get pregnant?
A: No! When a female has a period it means she didn’t get pregnant last month. It doesn’t say anything about what will happen in the month to come. Having intercourse during a period is NOT a way to prevent pregnancy (or STDs for that matter). Some females are fertile (able to get pregnant) during the periods and some aren’t. There isn’t any easy way to tell which females are fertile during their periods and which aren’t. Also, teenage females rarely have perfectly regular menstrual cycles. They can vary quite a bit in length, which means sometimes they are fertile during their periods and sometimes they aren’t.
As you can see, periods aren’t a way to determine when it’s a “safe” time to have intercourse if a couple does not want to get pregnant.