When a man has sexual intercourse with a woman, he places his penis within her vagina and moves his penis back and forth, typically in a thrusting motion, while the woman moves her hips. The vagina is warm and somewhat moist, and it places pressure on the man's penis. These sensations, combined with the friction caused by the in-and-out movements, stimulates the penis, which causes the man to have an orgasm and to ejaculate. If the woman is using birth control this will usually not cause her to become pregnant. Otherwise, the sperm in the semen can travel into the woman's uterus and Fallopian tubes. If there happens to be an ovum or egg cell in a Fallopian tube, a sperm may fuse with the ovum. This is called fertilization, and makes the woman pregnant.