How likely is it to get pregnant if you have sex while your on your period?

Unlikley: Depends on the cycle length. Sperm could potentially be viable and effective for up to 6 days inside the body. If a woman has a cycle length of 21 days then she will ovulate about cycle day 7. If she has a five day period, then there is the small possibility that having sex during the period on or after cycle day 2 could result in a conception. But, with a 28 day cycle there would be no chance.