A short menstrual cycle can be the result of a wide variety of conditions. MedGuidance.com reports that any significant change in estrogen levels can cause variation in the length of the menstrual cycle, including normal factors such as stress, travel, sudden weight loss or gain, certain forms of birth control, urinary tract infections or pregnancy.
WebMD emphasizes that short menstrual cycles and missed periods are experienced by many women from time to time, and it encourages women to focus on overall health, well-being and relaxation in order to promote the return of normal cycles. Both the Mayo Clinic and WebMD encourage calling a doctor in the event of menstrual cycles lasting less than 21 days or longer than 35 days.
WebMD points out that cycles shorter than 21 days or as long as 40 days can be normal for teens and that teens can expect cycles to become more regular as they get older. It adds that the onset of menopause can also be a cause of irregular menstrual cycles. While irregularities in the length of the menstrual cycle might be a result of normal life changes in women of ages 20 to 40, the Mayo Clinic describes several medical conditions that can also cause irregular cycles, including: polycystic ovary syndrome, pelvic inflammatory disease, uterine fibroids, eating disorders such as anorexia nervosa and premature ovarian failure. The Mayo Clinic suggests keeping track of one's periods in order to determine what is normal for each individual and stresses the importance of regular pelvic exams. It further notes that birth control pills can help some women regulate their menstrual cycles.