A short menstrual cycle, which is defined as one that lasts fewer than 21 days, typically signifies that ovulation has not occurred or that it has occurred early in the monthly cycle, according to Shady Grove Fertility. Shorter cycles become common as women age and the available number of eggs in the ovaries decreases. This causes the brain to release additional hormones, resulting in early or absent ovulation.
WebMD explains that shorter cycles can be caused by certain medications, birth control pills, hormone fluctuations and perimenopause, a condition that occurs prior to menopause that causes irregular menstrual cycles. In most cases, it is normal for women of all ages to experience an irregular menstrual period from time to time, but it is a good idea to consult with a physician if symptoms persist.
According to the Mayo Clinic, uterine fibroids are non-cancerous growths that develop in the uterus that cause abnormal, heavy bleeding and bleeding between periods, mimicking a shorter cycle. It is important for women to consult with a health professional if their menstrual cycles last fewer than 21 days or are more than 35 days apart. The Mayo Clinic also advises consulting a physician if the menstrual period lasts for 7 days or more, is accompanied by severe pain or stops for longer than 90 days.