All together now...

[babybloodheart.livejournal.com]

What is it that results in women whom live together or have close relationships together synchronies with each other's periods?

Comments

[mactavish.livejournal.com]

There's no real proof that it happens. Some scientists have "proven" it does, but not conclusively. But some people insist it does.

http://www.straightdope.com/classics/a2_306.html