If you think that having sex during your period is safe, think again! The timing for ovulation could prove to be unpredictable, even if you’ve got your cycle memorized by heart. Unprotected sex will always present the possibility of unplanned pregnancy, so why take the risk? Choose to stay safe with contraception, every time

Reference: http://www.mayoclinic.org/healthy-lifestyle/birth-control/in-depth/birth-control-pill/art-20045136?pg=1/