Half way through the monthly cycle, an egg cell is released from one of the ovaries. It moves slowly along the oviduct, towards the uterus. During sex, the erect penis moves up and down inside the vagina until ejaculation happens. About two teaspoonfuls of semen, containing millions of sperm cells, is left in the top of the vagina. The sperm cells swim through the cervix, through the uterus and along the oviduct. Only a few hundred make it as far as the egg. If the nucleus of one sperm gets into the egg and joins the egg's nucleus, fertilization has happened. The fertilized egg starts to divide and grow into a ball of cells, which moves down the oviduct into the uterus. The ball of cells settles into the thick lining of the uterus and begins to grow into a baby. This is the start of pregnancy.