Why do periods synchronise?

[iheartruby]

Why, when women spend time together, do their periods often end up synchronised?

[loulou06]

dont quote me on this but i thought it was something left over from prehistoric times when we all lived as clans in caves and stuff. this way all the women would be fertile at the same time and this would be convienent for the men to spread their seed to multiple partners and so safeguard the survival of the species.

[smudge]

Can just imagine it - Betty & Wilma.....

[wickedzoot]

someone once told me it has something to do with the moon and the tide......hmmm