After sperm are deposited into the upper vagina via ejaculation, they must travel through the cervical mucus into the uterus and then into the fallopian tube before they can meet with the egg. Sperm are transported on this long journey by their own rapid forward movement assisted by upward contractions of the uterine walls. During the trip, sperm prepare themselves to meet the egg by subtle alterations of their heads and movement patterns.
When they meet the outer membrane of the egg, the sperm start to burrow through it and then enter the egg itself. At the moment the first sperm successfully penetrates the egg, a reaction is triggered that makes the egg resistant to all other sperm. This single sperm absorbs into the egg, where the genetic material contained in its head fuses with that of the egg. Fertilisation is now complete.
The egg maintains its ability to be fertilised for about 12 hours after ovulation. Sperm can remain viable in the cervical mucus for 48-72 hours or more around the time leading up to ovulation.