If the menstruation is delayed for 20 days, but pregnancy is completely excluded. There are other causes of menorrhagia, such as progesterone, emergency contraceptive pills, etc., which can delay ovulation and therefore cause delayed menstruation. 2. Trans-uterine surgery, such as abortion or curettage, which causes adhesions in the uterine cavity, resulting in no discharge of menstrual blood. 3. Psychological factors, such as excessive stress or a recent change of environment, may cause delayed menstruation. 4, Endocrine disorders, such as obesity or weight loss, or recently staying up late, may cause delayed menstruation. 5. Chronic diseases, such as abnormal thyroid function, hyperthyroidism, hypothyroidism, and severe anemia, may also cause delayed menstruation. Therefore, if your period is delayed for 20 days, you may need to come to the hospital for an ultrasound to see the thickness of the endometrium and to have a blood test for endocrinology.