The normal menstrual cycle length is 25-35 days. Menstrual periods that come either at longer or shorter intervals of time would be considered abnormal. Many women aren't sure how to define the first day of the cycle for instance, should you count the first few days of spotting? While there is no absolute agreed-upon definition for determining the first day of a period, most physicians recommend that the first full day of menstrual flow be considered the first day. Another area of potential confusion is that some women measure their cycle length from the last day of one menstrual period to the first day of the next cycle. This would underestimate the cycle length. The correct way to determine the cycle length is from the first day of one period to the first day of the next menstrual flow. Polymenorrhoea is bleeding associated with menstrual cycles that are shorter than 21 days like in your case. It may be caused by hormonal or ovulatory problems. Ovulatory abnormalities that can cause frequent and/or heavy menstrual bleeding include uterine fibroids, polyps or tumors. Irregular growth of the endometrium caused by hormonal irregularities is the most common cause of heavy menstrual bleeding. Complications of pregnancy, including miscarriage, can also cause heavy bleeding. Many of the underlying causes of menstrual disorders cannot be prevented. However, women can reduce the risk of some causes by maintaining a healthy lifestyle, including eating a balanced diet and getting regular exercise. Other beneficial steps include avoiding recreational drug use, excessive alcohol consumption and cigarette smoking, assessing areas of stress or conflict in life.