A girl, after she reaches puberty is barred from entering the prayer room or performing any religious prayers during her menstruation. The only reason mentioned in our scriptures is that she is (ashudha) unclean during that period.
During ancient days, women would take bath in the river, pond or near the well. During the time when the girl would be menstruating, it would be awkward to bathe outside in the river as it would probably ‘pollute’ the water.
So, the ladies did not take bath. Since the ladies did not take bath, they were naturally termed ‘unclean’ during that time. Hence, their being barred from the clean’ places in the house, namely the prayer room or the kitchen.
However, Science has a different meaning for the menstruation. As per science, a woman is more irritable during those days because of a hormonal imbalance. Psychics claim that there is a different aura around a menstruating woman, maybe again a chemical reaction.
Segregating to pray requires a positive attitude and people tend to respond to the negative vibrations of a person standing with you; maybe that was the reason why a woman was barred from public places of prayer during these days.
However, the change in the old attitude and practice of keeping menstruating women out of places of worship (including temples) is therefore long overdue, considering as well that the Mimamsa (the Vedic / Hindu philosophy on rituals and traditions etc.) also advocates the immediate abandonment of traditions, customs and rituals which are now obsolete and outdated and appear to be detrimental to society presently, even though they might have had some positive aspect / use in the past and long ago.
