The process of fertilization begins with the union of an egg and a sperm. Sperm
enter the vagina during sexual intercourse. Moving up the vagina, sperm penetrate
the cervix and travel upward through the uterus into the fallopian tubes. In the upper  part of the fallopian tubes, a sperm may reach and penetrate an egg that has matured and been released from an ovary (called fertilization). The fertilized egg continues down the fallopian tube and implants itself in the lining of the uterus (the process known as conception).