The average length of the menstrual cycle is 28 days, starting with the first day of menstrual bleeding. When a young girl first begins menstruating, her cycle can be anywhere from 21 days to 45 days long. Normal menstrual cycles vary from woman to woman; for some, it might be as short as 21 days or as long as 35 days. For a healthy woman who has a history of regular menstruation, an occasional long menstrual cycle probably is no cause for alarm. If a woman's menstrual cycle is consistently longer than 35 days, she should see her doctor.
The term for menstrual cycles that are longer than 35 days is oligomenorrhea. Women who are concerned about having a long menstrual cycle should keep track of their periods over the course of several months. The first day of menstruation is counted as day one, and the cycle continues until the day before the next period begins. Some women find it helpful to keep a cycle calendar or use a menstruation resource online to help them track their periods.
If a woman has not gotten her period in 90 days or more, she might be diagnosed with amenorrhea. Amenorrhea can be caused by many things, including pregnancy, breastfeeding, perimenopause, eating disorders, major weight loss, too much exercise or severe stress. Female athletes and women who use steroids are at a higher risk for long or absent menstrual cycles. A long menstrual cycle also can indicate an underlying medical condition such as a pituitary tumor or thyroid disease.
One serious cause of a long menstrual cycle is polycystic ovary syndrome. When a woman has polycystic ovary syndrome, small cysts develop in the ovaries. Her body also produces excessive amounts of androgens, which sometimes are referred to as male hormones. A woman could experience severe complications, including infertility, if the illness is not treated. Irregular or missed periods are a key sign of polycystic ovary syndrome, so it is crucial for a woman to seek medical advice if she experiences abnormal menstrual cycles.
Treatment options for normalizing a long menstrual cycle vary depending on the cause. For young girls who have yet to develop a normal cycle, and for older women who are entering menopause, no treatment is necessary. If the abnormality is caused by a tumor, it might need to be removed. Some doctors will prescribe birth control pills to help regulate the menstrual cycle or hormones to restore balance.