most helpful answer no the sperm are just fine swimming through a little vaginal blood. vaginal blood will not prevent pregnancy but having sex during a true menstrual period means that you are not ovulating. if you are not ovulating you cannot conceive. sperm can be viable for a week or so trapped in vaginal or cervical mucous so having unprotected sex toward the end of your period is iffy so never count on this as a 100% method of birth control. some women will have vaginal bleeding mid-cycle (at a time of ovulation) so don't get fooled. for that extra-added insurance it is always wise for couples to use condoms.
does vagina blood kill sperm and prevents pregnancy?