A menstrual cycle of 30 days is very normal. A woman’s normal menstrual cycle is within 21-35 days, which is considered normal. A woman’s menstrual cycle is calculated from the first day of the last menstrual period to the day before the next menstrual period, which is a complete menstrual cycle. Women’s menstruation is controlled by the cerebral cortex and may be early or slightly delayed. Occasional fluctuations in menstruation, occasionally delayed within seven days or early within seven days, are normal fluctuations and do not require any treatment. If a woman has a menstrual cycle of less than 21 days, it is called frequent menstruation and is mostly caused by insufficient luteal function. If a woman has a menstrual cycle of more than 35 days and often does not have a period for 2-3 months, she is considered to have a menstrual disorder, mostly due to disease. The most common ones are polycystic ovary syndrome, hyperprolactinemia, and also thyroid disease, so relevant tests are needed to further clarify.