Fertilization is a process that usually happens within a few hours of ovulation, during ovulation, one of the ovaries releases an egg, which then travels down one of the fallopian tubes. If there’s sperm present in a woman’s fallopian tube when this happens, the sperm could fertilize the egg. The egg changes so that no other sperm can get in, once fertilized.
At the instant of fertilization, your baby’s genes and sex are set. If the sperm has a Y chromosome, your baby will be a boy. If it has an X chromosome, the baby will be a girl.
Usually, an egg has about 12 to 24 hours where it can be fertilized by sperm. However, sperm can live for several days in a woman’s body. Therefore, when the ovary releases the egg, sperm that are already present from intercourse a few days before could fertilize it. Or, if a woman has sex during the time the egg has been released, the sperm could fertilize the just-released egg.
See video on Fertilization