According to WebMD, fertilization takes place in the woman's fallopian tube. About two weeks after a woman's period, an egg is released. It stays in the fallopian tube for approximately 24 hours, waiting for fertilization by a single sperm.
WebMD explains that if an egg is not fertilized, it leaves the fallopian tube and moves into the uterus, where it disintegrates. If the egg is fertilized, it immediately changes chemically so that no other sperm can enter it. The fertilized egg then stays in the fallopian tube for the next three or four days. It begins dividing into multiple cells after about 24 hours and continues dividing as it moves. Once it arrives in the uterus, it implants into the lining of the uterus.