Fertilization begins when a sperm penetrates an oocyte (an egg) and it ends with the creation of the zygote. The fertilization process takes about 24 hours.

A sperm can survive for up to 48 hours. It takes about ten hours to navigate the female productive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the upper part of the vagina, only 1%, 3 million, enter the uterus. The next step is the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced. Penetration of the zona pellucida takes about twenty minutes.

Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization.