Obviously having a period but no blood is a case of menstrual hypomenorrhea. The normal menstrual volume is 20-60ml, if it is less than 5ml, it is called hypomenorrhea. The causes of hypomenorrhea are: 1. Endocrine disorders. If a woman’s long-term physical condition is weak, coupled with heavy bleeding after delivery or abnormal thyroid function, can cause endocrine imbalance resulting in low menstrual flow. 2, endometrial damage. If a woman does not clear her uterus in time after an abortion, or if there are more abortions, the endometrium is damaged or adhesions occur in the uterine cavity, which can cause a low menstrual flow or even amenorrhea. 3. If the ovaries are not functioning well, the level of estrogen decreases, the endometrium cannot grow thicker and the menstrual flow is low. 4.