Having unprotected intercourse at any time is risky, even when you're on your period. This include getting pregnant while in your bloody days.
According to a gynecologist, it is completely possible to conceive during your period if you are not using protection and not on any birth control.
This is because not all women have regular periods and the more irregular the period, the harder it is to gauge when ovulation begins and ends.
Typical menstrual cycle is about 28 days long, but some women have cycles as short as 22 days. If you have a shorter cycle, you could ovulate just a few days after your period ends.
And since sperm can survive in your reproductive tract for up to six days, if you engage in intercourse during your period, it's possible that the sperm may still be hanging around when you ovulate, ready to fertilize the egg.
So, apart from being more messy and less desirable, making love during these days will likely result to a future you.
Source: American Pregnancy Organization