Millions of sperms are deposited into the vagina during sexual intercourse. The sperms make their way through the cervix into the uterus and then on to the fallopian tubes. As they swim along this way their numbers decline. Only a few hundred sperm will get close to the egg. During the trip, sperm prepare themselves to meet the egg by subtle alterations of their heads and movement patterns.
Once inside the fallopian tube, the sperm attracts the egg by releasing a chemical. The egg is surrounded by a protective covering called the zona pellucida, which allows only one sperm to penetrate it. Once inside the egg, the head of the fertilizing sperm releases its genetic contents, which fuses with the nucleus of the egg. Fertilisation is now complete.
Sperm are able to survive for 2-3 days within the female's reproductive tract. The length of the time that a woman's egg can be fertilized by a man's sperm ranges from 12-24 hours.