 [ Symptoms ] i had a vestibular injury do not know what to do anymore [ Disease ] 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 do not get fooled for that extra added insurance it is always wise for couples to use condoms