What triggers puberty is still unknown; however, there are distinct changes in the hypothalamic-pituitary axis associated with the onset of puberty. These changes result in an increase in gonadotropin-releasing hormone (GnRH), which stimulates increases in leutenizing hormone (LH) and follicle stimulating hormone (FSH), hormones are responsible for the development of secondary sex characteristics. In males, LH stimulates testosterone production and FSH stimulates gametogenesis. In females, LH stimulates ovarian cells to produce androgens and progesterone and stimulates ovulation and FSH increases estrogen production (Neinstein, 1996).

Development of secondary sex characteristics is measured through the use of a Sexual Maturity Rating (SMR) scale, sometimes called the Tanner scale. Because initiation and rate of growth during adolescence are so variable, age is a poor indicator and predictor of change. SMRs are essential in evaluating adolescents and provide a method to gauge developmental progress. SMRs use a five-point scale of measurement. For females, ratings measure breast development and pubic hair growth, and for males they measure genital development and pubic hair growth.

As the age of onset of puberty varies, so too, does the length of time in each stage. Female pubertal development begins on average at 11.2 years of age (range 9.0 - 13.4) and lasts about 4 years. Breast budding is most frequently the first physical sign of puberty. Breast development and pubic hair development can progress at different rates and be at a different SMR level at any given time. On average, adolescent females'growth spurt starts about one year before breast development begins. Peak height velocity is reached about one year and one month after breast development begins. Menarche typically occurs one year after peak height velocity is reached, usually at SMR 3 (19%) or 4 (56%) at the average age of 12 years 4 months (range 9-17). See Table 1 for SMR parameters of female breast and pubic hair development (Tanner, 1962).

Male pubertal development begins on average at 11.6 years of age (range 9.5 - 13.5). Testicular enlargement is most commonly the first physical sign of puberty in males. SMR levels for pubic hair development and genital development (testes, scrotum and penis) can differ at any given time in the same individual. See Table 1 for male SMR parameters. The average length of time for completion of puberty in males is three years. Spermarche usually occurs early in puberty during SMR 2.5 (Tanner, 1962).