The absence of menstruation at the age of 18 is usually an abnormal condition. The onset of menstruation marks the onset of puberty and generally occurs between the ages of 12 and 16. At the age of 18, when the functional development of all organs tends to be perfect, if the first menstruation still does not occur, it should be considered that it may be due to disease factors, including congenital developmental abnormalities, chromosomal diseases or other factors. 1. congenital developmental abnormalities: there may be congenital developmental abnormalities of the uterus and adnexa, such as hymenal atresia, congenital absence of the uterus, infantile uterus, etc., which cannot support a normal menstrual cycle due to incomplete development of the reproductive organs; 2. chromosomal diseases: including Turner syndrome The chromosomal disorders: including Turner syndrome and androgen insensitivity syndrome. Turner syndrome, congenital ovarian hypoplasia, is a chromosomal disorder caused by complete or partial deletion of the X chromosome, which can also cause the appearance of no sexual signs and no menarche at puberty. Androgen insensitivity syndrome is an X-linked recessive genetic disorder, due to abnormal hormone secretion, women can have primary amenorrhea; 3, other factors: long-term adverse emotional effects, such as stress, anxiety, depression and other induced hormonal imbalance, long-term dieting or anorexia nervosa, because of the frequent use of contraceptives, adrenocorticotropic drugs and other drug-related amenorrhea, etc., can appear at the age of 18 years still no The first menstruation may not occur at the age of 18.