If a woman has sex 2 days after a period, the chances of a pregnancy
is very unlikely. A woman needs to ovulate
(release an egg) from her ovaries which needs to get fertilized for a pregnancy to occur.
Follicles start growing at the end of the periods and become mature and ovulates at around the 14-16th day after the periods. So these are your most fertile days.
Sex from the 10th day after the period to the 18th day has the strongest chance of causing a pregnancy.