It may be normal for menstruation to change from 7 days to 3 days, but it may also be due to endocrine abnormalities, thinning of the uterine lining, and decreasing hormone levels. 1. Normal phenomenon: Normal menstrual period is 2~8 days, menstruation from 7 days to 3 days is also within the normal range and is also a normal phenomenon. 2. Endocrine abnormality: when suffering from ovarian hypoplasia, polycystic ovary syndrome and other diseases, it may lead to endocrine abnormality, affecting the normal menstruation, and the menstruation may change from 7 days to 3 days. 3. Thinning of the uterine lining: Some women undergo repeated abortions, which may cause damage to the uterine lining, resulting in thinning of the uterine lining and shortening of menstrual days. 4. Decrease in hormone levels: When women reach perimenopause, due to the decrease in hormone levels, the menstrual cycle will be disrupted and the amount of menstruation will be reduced, so it will be manifested in the phenomenon of menstruation changing from 7 days to 3 days. It is recommended to go to the hospital in time, after the doctor’s interview, combined with other symptoms and related examination results, to clarify the cause of the disease, and follow the doctor’s instructions, the right treatment.