Once a couple has sex the woman doesn't automatically get pregnant on the day they had sex.
It can take up to a week for the sperm and egg to join up and fertilize. Then after that week it can take another 6 to 7 days for the fertilized egg to completely get implanted in the women's uterus which causes the woman to get pregnant.
So it can take at least 2 weeks to actually become pregnant with a baby after having sex.