There are several different reasons for menstruation to be a week early every month: 1. Consider normal physiological reasons, everyone’s menstrual cycle is different, some people are about 28 days, some people may be about 35 days. The length of menstruation is different due to individual differences, so a week early or a week late is normal, basically you can not worry too much, as long as their menstrual cycle is more regular. 2, consider the endocrine disorders and other problems caused by many women in the usual taking birth control pills or take the ring, may lead to early menstruation and other problems, so pay attention to keep observing their own 3, women from about 40 years old ovarian function began to gradually decline, so it may lead to early menstruation.