whats the chance of getting pregnant while having sex on menstrual periods????

about the same when your off ur period!

Looking at it mathematically,

Periods take place after 28 days on an average

But it takes 24 days for an egg to fully develop so that it can get fertilized. Complete egg remains in vagina for around 4 days. 

So, taking probability, 4/28 gives 1/7. That means if we have sex 7 times, there is a chance that 1 time, she would get pregnant.

Again, when Bleeding is going on, either the egg is already taken out of body or the blood vessels providing nutrients to the egg are gone. So in both ways, you will not get pregnant.(most likely) hope this helps<3