If you've been menstruating for six days, but the last few months how come only three days huh? There are several possible causes, ranging from natural, lifestyle changes to the possibility of certain diseases. What you need to know from the beginning, each woman's menstrual period can be different. Menstrual periods 2-7 days with menstrual cycles 21-35 days are still classified as normal. Although three days seem short, but if menstruation occurs regularly, then it can still be classified as normal. Natural Causes of Shorter Menstruation Short natural menstrual periods can indeed occur at certain times of a woman's life. Short menstrual periods, about 3 days, more experienced by adolescents or the elderly (elderly) who are approaching menopause. This can happen because this group of women does not already have sufficient estrogen levels. Estrogen is what forms the endometrium which decays every menstruation. During puberty, hormone levels can vary so the length of t