(THE DAILY COLLEGIAN ONLINE) 8 October 2002:

While there is no evidence conclusive enough to prove the theory, some scientists believe that women who live in close quarters or who spend a lot of time together develop menstrual synchrony, which means they tend to menstruate around the same time of the month.

Studies by Leonard and Aron Weller found that menstrual synchrony occurs between co-workers and between women in some closely knit family units.

Other studies found a greater degree of menstrual synchrony among women who were close friends than among women who did not have close relationships, she said.

A study that examined a women’s basketball team for an extended period of time found no correlation between the women’s menstrual patterns.

Some scientists attribute pheremones to this phenomenon, others say it’s because of similar lifestyles or use of oral contraceptives.  While they disagree on cause, most scientists that have studied the phenomenon acknowledge that synchronized menstruation does exist.