Is it more or less likely to get pregnant while on your period?

Less likely, but still possible.

A woman can only get pregnant if she has sex either during ovulation or up to a week before ovulation when there is fertile quality cervical mucus present - this helps keep the sperm alive for up to 7 days and helps it to swim up through the cervical opening.

During an average 28 day cycle a woman will be fertile between days 7-16 (ovulation on day 14, there can be a viable egg present for up to 48 hours) - by the time a woman menstruates the egg from the last cycle is long dead, and she cannot ovulate during menstruation. However if a woman's menstrual cycle is short she may ovulate soon after menstruation, thus if she was to have sex during her period sperm could survive up to 7 days, by which time she may have ovulated and thus she may get pregnant.