Menstruation is one of several changes in puberty related to the development of primary and secondary sex characteristics. Primary sex characteristics are associated with the development of the organs and body structures related directly to reproduction. Secondary sex characteristics are the visible signs of sexual maturity that do not involve the sex organs directly. In girls, developing primary sex characteristics involves changes in the vagina and

uterus. Secondary sex characteristics include the development of breasts and pubic hair. Breasts begin to grow around age 10, and pubic hair appears at about age 11. Underarm hair appears about 2 years later. For some girls, signs of puberty start unusually early. One out of seven Caucasian girls develops breasts or pubic hair by age 8. For African American girls, the figure is one out of two. The reasons for this earlier onset are unclear, and what defines normal and abnormal onset is a controversy among specialists (Ritzen, 2003; Mensah et al., 2013; Mrug et al., 2014).

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