Here we go, I found it on twilightlexicon.com from one of SM's interviews with them (Personal Correspondance #2):
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).