If vampires go crazy with the smell of bloody, how did the Cullens tolerate Bella when she had her period? A small cut caused Jasper to attack.
The author, Stephanie Meyer covered this in an interview for the Twilight Lexicon. According to her, menstrual blood does not have the same potency as "fresh" blood and Edward is (thusly) able to control himself better.
Meyer: Edward can’t be around Bella when she’s bleeding without being in intense temptation–to the point of pain. A few drops would only make it worse. Like a few drops of water when you’ve been wandering in the desert for days… You’d totally suck down the whole gallon of Evian.
Several girls wanted to know if Edward would have a more difficult time being around Bella when she’s having her period. Answer: Yes, a little bit, but he would never say anything about it – much too much of a gentleman. And Bella would be way to embarrassed to ask. (It’s not the same as a cut, though. It’s sort of "dead" blood, if you get my meaning).