Delayed menstruation for 21 days and testing for non-pregnancy may be due to physiological factors such as poor living habits and mental stress, or it may be caused by diseases such as early onset ovarian insufficiency and polycystic ovary syndrome. 1. Physiological factors: (1) Poor living habits: if a woman has poor living habits, often stays up late at night, and is overworked, it may lead to irregular menstruation and a situation where the menstruation is delayed for 21 days to test for non-pregnancy. (2) Mental stress: when there is a lot of mental stress, emotional anxiety and tension in life, it may lead to endocrine abnormality in women, which may lead to irregular menstruation, resulting in a situation where menstruation is delayed for 21 days to detect non-pregnancy. 2.Diseases: (1) Early-onset ovarian insufficiency: chromosomal abnormalities, autoimmune diseases, medical injury and other factors may lead to early-onset ovarian insufficiency in women, and patients often show scanty or frequent menstruation, and may even have amenorrhea, which leads to the situation that menstruation is delayed for 21 days to detect non-pregnancy. (2) Polycystic Ovary Syndrome (PCOS): A combination of genetic and environmental factors may lead to the development of polycystic ovary syndrome (PCOS) in women. Patients often have scanty menstruation, low menstrual flow or amenorrhea, which can also be manifested as 21 days delayed menstruation test without pregnancy. It is recommended that if the patient is not pregnant after 21 days of delayed menstruation, she should consult a doctor in time to clarify the cause of the disease and strictly follow the doctor’s instructions to carry out targeted treatment.