Susan, There are several factors which affect time of menstruation. The menstrual cycle mirrors (in it's healthy state) the moon cycle. Even artificial light affects the menstrual cycle. Diet is another huge factor. For a better understanding of why women in the same household tend to menstruate at the same time, I highly recommend Christiane Northrup's book Women's Bodies, Women's Wisdom. The dynamics which are occuring in the household have a dramatic impact on menstruation. For example, I come from a family of my mother, father and one sister. My father is very controlling and manipulative. During the time that the three of us were all living with him, we all menstruated at the same time and had severe dysmenorrhea and very heavy bleeding. Besides the fact that none of us should have been eating gluten or casein (which affect the uterus), our relationship with that man was also affecting our menstrual status. We were all bleeding during the waxing quarter moon, the least common, least desireable time to bleed. Since leaving that home and eventually going gf/cf, I am also on a 25-31 day cycle, bleeding between the waning full moon and the dark moon. Much better! For a list of menstruation links, see my web site's moon page. ~~~~~~ Stacie http://www.tolen.org (follow the cave man!) _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. Share information about yourself, create your own public profile at http://profiles.msn.com.