The normal menstrual cycle varies from 21 to 35 days and the duration of flow ranges from two to seven days, with an average of four days. A woman may have some variation in the characteristics of her cycle, especially just after she starts having periods and for a few years before menopause. However, between the ages of 20 to 40 most women report fairly consistent cycles from month to month. If a woman has a significant change in her periods, she should have a complete evaluation by her doctor to determine the cause.
Many different factors regulate the menstrual cycle, so changes can result from a variety of causes. A significant decrease in flow can occur if there is an imbalance in the reproductive hormones. This might occur with intense physical training, weight loss and stress. Much more common is the expected decrease in flow associated with the use of oral contraceptive pills. Damage to the uterine lining from an infection or after uterine surgery is a rare cause of decreased menstrual bleeding.
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