What is the reason for the 21 day cycle?

Normal women's menstrual cycle is twenty-one days to thirty-five days. If each month is twenty-one days, consider it a normal phenomenon. But if it appears in recent months, it is abnormal, and there may be irregular menstruation. The causes are more, such as premature ovarian failure, uterine polyps, endometriosis, uterine fibroids, etc.