There is nothing wrong with having sex while a woman is on her period, but the re are some details about this situation you (and she) might want to know first. 1. The Basic Biology Sex ed. told you the basics about a woman's menstrual cycle, but here's what's important to know for this situation. A period marks the end of the body's monthly attempt to get pregnant. Awaiting a fertilized egg, the uterus developed a thickened lining to keep this potential occupant happy. If an egg doesn't get fertilized, the body can't sustain this special womb, so hormone levels drop and the lining is shed. "The bleeding is because there is a blood supply that went into those tissues, and it bleeds until the tissue comes out enough and the blood vessels start to close off," says Rowen. 2. What it Feels Like Women may already be in a considerable amount of pain during their period, so sex may seem like it would add to that. The short answer is: it shouldn'