A autora, Stephanie Meyer, falou sobre isso em uma entrevista para o Twilight Lexicon . Segundo ela, o sangue menstrual não tem a mesma potência que o sangue "fresco" e Edward é (assim) capaz de se controlar melhor.
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).