Several recent studies suggest that the timing of the onset of puberty in girls has become earlier over the past 30 years, and there is strong evidence that the increasing rates of obesity in children over the same time period is a major factor. This article reviews studies from the United States that examined the age of menarche and the age of onset of breast development and pubic hair as a function of body mass index, which is a good surrogate measure of body fat. These studies are nearly all crosssectional, so many questions remain unanswered. However, at least several studies show that girls who have relatively higher body mass index are more likely to have earlier menses, as well as a relationship between body mass index and other measures of pubertal onset. The evidence published to date suggests that obesity may be causally related to earlier puberty in girls rather than that earlier puberty causes an increase in body fat. In contrast, few studies have found a link between body fat and earlier puberty in boys. A growing body of evidence from both rodent and human studies suggests that leptin may be the critical link between body fat and earlier puberty. Leptin-deficient mice and humans fail to enter puberty unless leptin is administered, and rodent studies indicate that very low levels of leptin stimulate gonadotropin secretion both at the hypothalamic and the pituitary level. Current evidence indicates that leptin appears to play a permissive role rather than act as the critical metabolic signal initiating puberty. The linkage between body fat and the reproductive axis in girls may have evolved in mammals as a mechanism for ensuring that pregnancy will not occur unless there are adequate fat stores to sustain both the mother and the growing fetus. HISTORICAL PERSPECTIVESRecords from several northern European countries, particularly Norway, Denmark, and Finland, document that the age of menarche, a convenient marker for the timing of puberty in girls, decreased from ϳ16 to 17 years during the 19th century to ϳ13 years by the middle of the 20th century. 1 In the United States, a decline from 14.75 years in 1877 to just under 13 years by the period of 1950 to 1970 has been reported. It has been widely assumed that improved health and nutrition associated with the coming of the Industrial Revolution were responsible for most if not all of that decline in the mean age of menarche. At the very least, these historical data suggest that the timing of puberty is not solely genetically determined but can be influenced by epigenetic factors. There have also been studies to suggest that obese girls tend to mature earlier than normal and that "thin" girls tend to mature later. For example, a significant delay in puberty and menarche is seen in girls who are very physically active and have markedly diminished body fat. 2 After reviewing longitudinal growth data for 181 normal girls, Frisch and Revelle 3 observed that the mean weight at menarche was constant at ϳ48 kg for girls who reached menarche before age 1...

The results are consistent with obesity's being an important contributing factor to the earlier onset of puberty in girls. Factors other than obesity, however, perhaps genetic and/or environmental ones, are needed to explain the higher prevalence of early puberty in black versus white girls.

In 1997 a study from the Pediatric Research in Office Settings network, based on pubertal staging of >17,000 girls between 3 and 12 years of age, indicated that breast and pubic hair development are occurring significantly earlier than suggested by our current guidelines, especially in African-American girls. In response to this article, the Lawson Wilkins Pediatric Endocrine Society undertook a comprehensive review of this topic. The primary conclusions of this review are: 1. The current recommendation that breast development before age 8 is precocious is based on outdated studies. Until 1997, no data were available on pubertal staging in US girls that could have documented a trend to earlier maturation. 2. The 1997 study indicates that stage 2 of breast and pubic hair development is being achieved ~1 year earlier in white girls and 2 years earlier in African-American girls than previous studies have shown. 3. Concerns that girls with moderately precocious puberty will be significantly short adults are overstated; most have adult height within the normal range. 4. Therapy with gonadotropin-releasing hormone agonists has not been proven to have a substantial effect on adult height in most girls whose puberty starts between 6 and 8 years of age. 5. New guidelines propose that girls with either breast development or pubic hair should be evaluated if this occurs before age 7 in white girls and before age 6 in African-American girls. No changes in the current guidelines for evaluating boys (signs of puberty at younger than 9 years) can be made at this time.normal puberty, breast development, pubic hair.

In light of the above trends, the view that onset of any pubertal changes prior to age 8 years requires an extensive evaluation should be reevaluated. The majority of such early-maturing girls are normal girls at the early end of the age distribution for pubertal onset. As much attention should be paid to the rate of progression of pubertal findings as to their age of appearance.