I'm sorry, but if you have to ask, should you be having sex? You could either abstain from sex, or use one of the many contraceptives. The most popular are condoms. If she's already pregnant, there's the "plan B" pill that she has to take within 72 hours after the "incident". If it's been passed 72 hours, then you should research the abortion laws for your area, and take her to the clinic (if your state allows). The number one way is abstain from sex. The other dozens would be contraceptives. If your talking about after the sperm has already entered the woman there is a "after the fact" pill that can be taken but it has to be done within a certain amount of time. Why not practice safe sex at all times, then there are no issues. use a condom!!!