If you are eighteen years old and still do not have menstruation, it is often due to the following reasons: 1) atresia of the hymen, which can cause periodic pain in the lower abdomen, and a hymenotomy can be performed.2) congenital developmental abnormalities, such as congenital absence of the uterus, of course, will not have menstruation.3) abnormal development of the uterus, such as naïve uterus, the uterus is relatively long and narrow, and the inner lining is relatively thin, so it will not have menstruation either.4) the uterus will not have menstruation, so it will not have menstruation. It is advisable to have an ultrasound examination first, mainly to understand the development of the uterus. If the development of the uterus is okay, you can take a blood test for sex hormone six to understand the endocrine situation, to understand whether the ovarian function is normal, if it is a childish type, it will not menstruate again. If the development of the uterus is still normal, it is recommended to use medication for treatment, there will be the possibility of menstruation.