A recent article offers some insight on the evolutionary role of menstruation: The evolution of menstruation Emera D, Romero R, Wagner G., Department of Medicine, Washington University School of Medicine, St. Louis, MO, USA. http://www.ncbi.nlm.nih.gov/pubmed/22057551
Another reason is - the lining of the uterus is also a sensor for fetal quality, detecting chromosomal abnormalities and allowing them to be spontaneously aborted early. (from PZ Meyers, http://scienceblogs.com/pharyngula/2011/12/21/why-do-women-menstruate/)
Some say menstruation is a monthly vital sign. See Gloria Steinem's classic essay: If Men Could Menstruate http://www.mum.org/ifmencou.htm
answered Jun 10 '13 at 15:50
Well, I think if there is a Goddess, I think She designed us this way in order to release toxins from our bodies every cycle. The Chinese call a period "heavenly water," and they call the month after delivering a baby "golden month." They say that if a woman has many health problems before having a baby, then golden month is the time to fix them by proper nourishment and rest after a release of many toxins as well as the baby. I think periods are a mini version of this. I bet there is an equivalent in men, too.
answered May 25 '16 at 20:36