Although anorexia, pregnancy or the pill can interrupt it, and menopause or hysterectomy stop it, menstruation is an experience almost all women have had. Bodily changes associated with it include temperature, hormone levels, vitamin concentrations, pain threshold and even blood sugar. Visual and audio perceptions can get more or less acute at different points on the cycle. Many people experience psychic changes as well - for instance, feeling more inward-looking, or more sexy, at particular times of the month. Some people feel sick or have pains when they menstruate.
It might seem strange that such a continual influence on people's lives is so rarely talked about. But some of the most important influences on how we view menstruation come from earlier cultural beliefs about it. Ideas we've inherited include that menstruation is unclean and taboo, and should be kept secret.
An interesting parallel is with horse-meat. In some countries, it's no big deal to eat horse. But in England, people mostly wouldn't consider it, and may even have a feeling of revulsion at the idea. The explanation is that in early English religion, the horse was a sacred animal (hence the horses cut in the chalk hills in some places). Over time, that fear of its magical power has gradually transmuted into another kind of fear, more like disgust.
Similarly with menstruation, the original sense of sacred power around it has largely been forgotten, while the often uneasy sense of the taboo survives. Some languages still have this link explicitly. "... 'Taboo' or 'sacred' in Polynesian and Siouan is the same word as 'menstruating'. In Dakotan 'Wakan' means 'spiritual, wonderful, menstrual'." 
The link between power and taboo also shows up in puberty rituals from around the world. Some rituals for girls match those traditionally performed by holy people to protect themselves and the people around them from their mystical power. "... the girl is viewed as charged with a powerful force which, if not kept within bounds, may prove destructive both to herself and to all with whom she comes in contact. To repress this force within the limits necessary for the safety of all concerned is the object of the taboos in question." 
Menstrual magic also connects with the fear of old women as witches. Post-menopausal women were the wisest people because they kept their "wise blood". "... in the 17th century, Christian writers still insisted that old women were filled with magic power because their menstrual blood remained in their veins. ... The same 'magic blood' that made them leaders in the ancient clan system made them objects of fear under the new patriarchal faith." 
In ancient Mesopotamia, the Goddess made mankind from clay "and infused it with her 'blood of life' ... [hence] the name of Adam, from the feminine adamah, 'bloody clay', though scholars more delicately translate it as 'red earth'". 
Another theme connected with the idea of "wise blood" is that of menstruating women as visionaries or shamans. In some cultures, women would withdraw from society at menstruation to meditate. At this time they were believed to see especially clearly what they and their community needed. (Sometimes this ritual was reinterpreted: they were being kept out of the way because they were dangerous or unclean.)
One idea about PMT is that it results from a kind of clash - between the way that the world expects you to keep on doing things, while your instincts are telling you to be quiet and meditate. Alternatively, it might come from perceptions or dreams that are being suppressed - of changes you want in your life, or of unpleasant realities you don't want to face. Apparently, the closest men ever get to PMT is when they're deprived of their dream sleep.
Very little research has been done on women's experiences of their cycle. Even the physiological aspects of it aren't well understood. One reason is probably that in psychology, as in many areas, men's experience is generally taken as the norm. Seen from that paradigm, the menstrual cycle is an aberration - an inconvenient oscillation superimposed on a "normal" life.
By the same token, society isn't aligned around special time for women at menstruation. In most places, the expectation is that people will pretend it's not happening.
In the workshop at BiCon, we finished with the questions: "How would you design your ideal environment for the times before and during your period?" and "What changes could you make in your life to empower yourself to make the most of your own menstruation?".
Copyright Jennifer Moore 1996