why do women get their period at the same time if they spend a lot of time together?

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Answers:
Synchronous menstruation was first described in 1971 by researchers McClintock and Stern. It is a proven occurrence, borne out by additional studies. Though they are still unclear as to exactly why it occurs, the most widely accepted theory is that it is due to pheromones. Others think it's due to some evolutionary carry-over from prehistoric times, when it was common for men to take multiple mates to ensure the survival of the species, but the pheremone theory makes more sense.

Source(s):
McClintock, M. K. (2000). Human pheromones: primers, releasers, signalers or modulators? In K. Wallen and J. Schneider (Eds.), Reproduction in Context: Social and Environmental Influences on Reproductive Physiology and Behavior. MIT Press. Pp. 355-420.

McClintock, M. K. (1998). Whither menstrual synchrony? Annual Review of Sexual Research, 9, 77-95.

Other Answers:
Thats what I've heard, that we can pass chemicals to one another that cause us to have our periods at the same time

they spend a lot of time together.. i just asked that question.check it out!

i dont know but that is true we do , its funny.

do they! I guess thats why they all seem to get nasty at work at the same time!

Something with hormones and the dominant personality of the group. Its freaky and true. I have also noticed too that the older I get the more likely I am to fall into a younger females cycle. Is it telling me time to move over? It is true and is called "synchronous menstruation" and is phenomenon described first in 1971 by Martha McClintock.
It is pretty much a theory, but it is believe it is related to pheromones.
Check this out:
http://www.newscientist.com/backpage.ns?id=lw892 There is a theory that naturally occuring chemicals called pheromones are produced and released. The pheromones are scents which we are not conciously aware of but which can influence biological processes.

The theory is that women who live together or spend a lot of time in close proximity sense each others pheromones and their bodies will sometimes "synch" menstrual cycles.

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why do women get their period at the same time if they spend a lot of time together?

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