A baby is made when an egg cell made in the ovaries is joined (fertilized) by a sperm cell made in the testicles and implants in the uterus where it can grow into a baby. This usually happens through sex, where a penis is inserted into the vagina and the penis releases fluid called semen that contains sperm. The sperm travel through the cervix, the uterus then into the fallopian tubes. If an egg has been released around the same time, fertilization happens. The fertilized egg travels and attaches to the uterus and starts developing into a baby.