• Most of the time, a doctor can’t tell if a girl has had sex just from a pelvic exam (and doctors don’t usually give teen girls pelvic exams unless there’s a sign of a problem). But you should let your doctor know if you’ve had sex anyway. Why? Having sex puts a person at risk for STIs as well as unplanned pregnancy. When you tell your provider you’ve had sex, he or she can look out for signs of infection. Treating STIs early is important not just for your health, but also for the health of your partner. An STI that’s not treated could cause scarring of the reproductive organs and might lead to infertility (inability to have a baby) in men and women.