Menarche is the occurrence of a first menstrual period in the female adolescent. Menstruation is the monthly shedding of the functional layer of the uterine endometrial lining that occurs when ovulation is not followed by fertilization. It occurs approximately every 28 days, with a range from every 21 to every 45 days. The average age of onset of menarche is 12.4 years. Most menstrual periods last between 3 and 7 days, and menses that last more than 10 days is considered abnormal. Menarche signals maturation of the adolescent female body. It commonly is associated with the ability to ovulate and reproduce. However, the appearance of menarche does not guarantee either ovulation or fertility.[1][2][3]

Menarche occurs in the setting of a maturing hypothalamic-pituitary-ovarian (HPO) axis. It relies on the following processes: normal hypothalamic and pituitary function, normal female reproductive anatomy, normal nutrition, and the general absence of other intervening chronic illness. It is a marker of normal female reproductive health and wellness. Most females recognize menarche as their body’s critical declaration of fertility. The absence of normal menstrual periods, not related to pregnancy, is called amenorrhea. Primary amenorrhea is the complete absence of any menstruation by 15 years of age. Secondary amenorrhea is the cessation of menstruation for 3 months or more after it has started. In the first 2 to 3 years after the initial onset of menses, it is common for irregular cycles to occur, with adolescents often having several months of missed menses. Many of these irregular cycles may be nonovulatory due to poor early regulation of the hormonal interactions between hypothalamic, pituitary, and gonadal hormones. Tracking menstruation on a paper calendar or using a smartphone app can be helpful in determining if menstrual periods are becoming more regular for adolescents and also for predicting when ovulation is most likely to occur. The information can be used for both pregnancy planning and pregnancy prevention.[4][5][6][7]

The average age of menarche is 12.4 years but varies with ethnic or racial background. This age has been decreasing over the past 100 years and is most often attributed to improvements in child and adolescent nutrition. Exogenous hormones in the diet also may contribute to this earlier initiation of menarche. The average age of menarche for Caucasians generally is slightly later compared to other races. Menarche most commonly occurs in sexual maturity rating (SMR), or Tanner stage, IV. It is abnormal for menarche to occur before the appearance of secondary sexual development. Sexual abuse, genital trauma, tumors, or bleeding disorders should strongly be considered in the differential diagnosis of females who are prepubertal and experience vaginal bleeding. Menarche generally is considered early if it occurs before 9 years of age and late if it occurs at or after 15 years of age. Menarche is considered delayed if there is more than a 5-year lapse between the onset of breast development (SMR II breasts or breast buds) and the first menses. Menarche occurs during a time of puberty that is associated with rapid growth velocity. Typically, adolescent females who experience menarche are not ovulatory with every cycle in the first year. It is estimated that most will have ovulatory cycles within 5 years of menarche. Imperforate hymen is the most common congenital genital tract anomaly in females ranging from 1/1000 to 1/10,000 in prevalence. It can be noted at birth, as the estrogenized hymen in the newborn period often is readily apparent. The absence of a normal uterus or vagina, due to Mullerian agenesis or Mayer-Rokitansky-Kuster-Hauser syndrome, has an incidence of approximately 1/4500 females and often is not diagnosed until a patient presents with the absence of menarche. Other complex hormonal abnormalities such as androgen insensitivity syndrome can present with external female development and primary amenorrhea. Abnormal development of female gonads due to Turner syndrome also can lead to ovarian agenesis and absent menses.[8][9][10]

Menarche is the result of complex interactions between the hypothalamic, pituitary, and ovarian hormones. It also can be affected by thyroid, adrenal, and pancreatic hormones. Thyroid hormones are necessary for normal menses, and their deficiency or excess can inhibit menarche or lead to abnormalities in existing menstrual patterns. Abnormally elevated adrenal androgens or insulin can affect normal ovarian estrogen production, as well as decrease the normal pituitary production of luteinizing hormone. Low adiposity can inhibit normal menarche, and it is estimated that a minimum body fat of 17% is necessary for menarche, with 22% body fat necessary for maintaining normal menses. The hormone leptin appears to have a role in the maintenance of normal menstrual cycles as well. Stress and obesity appear to be predictors of early menarche.