I have heard that women who live together get their periods at the same time; some female friends of mine confirmed that this was true, at least in their case.
But if this is true, do women in large groups get their periods at the same time – all the women in an office building for example? After all, they may be working together in close quarters for over 40 hours a week, and may well be spending as much time in the office as they do at home among those they live with. Is this possible? And if it is possible, what about an even larger population set – say, all the women in a city with a high population density?