sexual intercourse is the natural and most common way to make a baby. it involves a man and woman having sex without birth control until the man ejaculates, or releases, semen from his penis into the woman's vagina. the semen, which has sperm in it (which is made in his testicles), moves to the fallopian tubes, and if it finds an ovum on its way, it will try to fertilize it. if this works, the now fertilized ovum sticks to the side of the uterus and the woman becomes pregnant. the fertilized ovum then develops into a human. medically, it is called a pregnancy until birth.