Fascinating question, I'm very interested in the answer. Now on the same topic, have they solved the problem of how a woman can PEE in free fall? Male astronauts have a tube thingie, but women have tended to have to use a catheter... ouch... what do they do now?
About the actual question, I would assume that the periods would be skipped by using birth control or similar pills. It is nowadays quite safe to do that, lower dosages and all that. In fact, some medical people are beginning to think that menstruating every month may actually be harmful, since Mother Nature indended us to be pregnant so much of the time, it might actually be safer to have fewer periods... but the data aren't in on that.
Do we have any members that are affiliated with the Space program?