The presence of large blood clots in menstruation may be a normal phenomenon, or it may be caused by reproductive system diseases, endocrine disorders, intrauterine device-induced adverse reactions, medication factors and so on. 1. Normal phenomenon: menstruation is the process of shedding of endometrium, which may be accompanied by a small number of clots, that is, coagulated blood and mixed with the endometrium, if the bleeding is not large, not many clots, the body has no obvious discomfort, generally a normal phenomenon. 2. Reproductive system diseases: such as endometriosis, uterine polyps, uterine fibroids, etc., which can increase menstrual flow and easily induce large blood clots in menstruation, and may be accompanied by lower abdominal pain, menstrual bleeding, prolonged menstruation, vaginal secretion abnormalities and other uncomfortable symptoms. 3. Endocrine disorders: can be caused by hyperthyroidism, hypothyroidism, hyperprolactinemia and other diseases, due to the imbalance of hormone levels in the body, can induce endometrial thickening, increased menstrual flow during menstruation, which can be mixed with large blood clots. 4. Adverse reactions induced by intrauterine device (IUD): the placement of IUD may trigger local inflammatory reactions, enhancement of fibrinolytic activity and other adverse reactions, resulting in the appearance of large blood clots during menstruation. 5. Drug factors: mainly for a variety of steroid hormone drugs, such as hydrocortisone, nil estradiol, etc., may make the drug user’s menstrual cycle disorders, resulting in the appearance of large blood clots in menstrual blood. Patients with large blood clots in menstruation, accompanied by other uncomfortable symptoms, should consult a doctor in a timely manner, under the guidance of the doctor to identify the cause of the disease, if necessary, follow the doctor’s instructions for appropriate treatment.