you actually can if you have a very short cycle, the typical 28-30 day rule does not apply, sperm can live for 5 days if you have sex the day befoe your period is due, your period is for 3-4
days and you have a very short
cycle, so ovulate the day after, the sperm is in your tubes and therefore will fertilise the egg and off we go, the same as having a short cycle and having sex while you have your period, some women ovulate durning the las t day or two of their periods so can get pregnant then