In the United States, menarche (onset of menstruation) typically occurs around age 12, although some youngsters start puberty when they are only eight or nine, others when they are well into their teens.
In some, a girl begins to wear the skirt after menarche; in others menarche is recognized by a change from a small leather panel skirt to a fringed skirt that wraps all the way around the body.
For girls, the hormone estrogen substantially increases, leading to breast development, a curvier figure, and menarche, which is the first period.
Girl puberty consists of many changes, from the beginning of menarche to the rapid social and cognitive developments taking place in the brain.
Regular ovulation and the ability to carry a baby to full term usually follow menarche by several years.
Secondary amenorrhea is defined as the absence of menstruation after menarche has taken place.
Primary dysmenorrhea usually presents during adolescence, within three years of menarche.
Chumlea, William Cameron, et al. "Age at Menarche and Racial Comparisons in U.S. Girls."
Once menarche (the first period) begins, you may feel frightened, shocked, and confused.
If it did not happen in Stage Three, menarche (first menstruation) should start now.