The sudden decrease in menstrual flow may be due to the following reasons:1. Ovarian hypogonadism, with age, especially at menopause, the gradual decline in ovarian function is the direct cause of the decrease in menstrual flow, which is a normal physiological phenomenon.2. Endocrine abnormalities, such as polycystic ovary syndrome, hyperprolactinemia, hypothyroidism, adrenocortical function abnormalities can lead to endocrine disorders, which can lead to a decrease in menstrual flow. This will lead to a decrease in menstrual flow. 3, weight-related reasons, if the body mass index is more than 28, significant weight gain or weight loss, fat content is insufficient to maintain normal menstruation will lead to a decrease in menstrual flow, and serious will lead to amenorrhea. 4, uterine operations lead to a decrease in menstrual flow, for example, abortion, purging and other uterine operations lead to the uterine cavity adhesion. 5, too much mental tension, suffered a major blow also can cause a sudden decrease in menstruation.