It is caused by dysplasia of the uterus as well as endocrine disorders. The endometrium proliferates and sheds under the regulation of oestrogen and progesterone, and after shedding it is discharged through the cervix and vagina to form menstruation.
When a patient has a dysplastic uterus with no endometrium or uterine cavity, or when the endometrium is too thin and does not respond to estrogen stimulation, no regular endometrial growth and shedding, no menstruation can be formed; when there is no vagina, menstruation cannot flow out and periodic abdominal pain can occur, so all these conditions can lead to the absence of menstruation.