Breast development and the first menstrual period are telltale signs of puberty in girls. The onset of puberty continues to come increasingly earlier — now happening before age 9 in some females. Parents, physicians, and researchers are puzzled by the cause of this early onset. According to a recent study, a high body mass index (BMI) may trigger the early onset of puberty, causing girls to develop breasts as early as age 8.

Publishing in the journal Pediatrics, researchers from the San Francisco Bay Area, New York, and Cincinnati wanted to examine the impact of BMI and race/ethnicity on the onset of breast development. More than 1,200 girls ranging from 6 to 8 years old were enrolled and longitudinally followed at regular intervals from 2004 to 2011 in the study. The researchers opted to do multiple regular visits for each girl as a means to have a good perspective of what happened to each girl when puberty occurred. The five stages of breast development, known as the Tanner Breast Stages, were used by the researchers as a method to establish criteria of pubertal maturation.

In the study, breast development varied by race, BMI, and geographic location. The reearchers found that breast development begins in white, non-Hispanic girls on average at 9.7 years old, while black girls experience this development earlier at 8.8 years old, NBC News reports. The average age for breast development in Hispanic girls was 9.3 years, and 9.7 years for Asian girls.

Previous studies have found that race as well as BMI played a role in predicting the onset of puberty, but this study confirms that "BMI supplants race as the No. 1 factor,” according to Frank Biro, lead author of the study and director of adolescent medicine at Cincinnati Children’s Hospital.

Girls whose BMIs were below the 50th percentile started to develop breasts at about age 10 while those in the 85th and 95th percentiles began to develop breast as early as age 8.5. These findings suggest that clinicians may need to reevaluate the ages for both early and late maturation in girls. Earlier maturation may bring an array of challenges such as lower self-esteem, higher rates of depression, norm-breaking behaviors, and lower academic achievement, wrote the researchers, Medical Xpress reports.

Girls who begin to develop breasts at an earlier age are at risk for several cancers, particularly breast cancer. According to the Breast Cancer Fund, this trend is speculated to be the result of exposures to endocrine-disrupting chemicals such as bisphenol A (BPA) and phthalates before puberty, when the reproductive system, including breast tissue, is extremely sensitive to low levels of estrogens.

In a similar study, early puberty in girls was found to be associated with the increased likelihood of developing type 2 diabetes in adulthood. Women who began to menstruate between the ages of 8 and 11 were 70 percent more likely to develop type 2 diabetes than those who began to menstruate at age 13 — the average age. The researchers also found that excessive weight often appears to play a role overall in the development of type 2 diabetes.

Although the studies suggest that a high BMI is an important factor in the early onset of puberty in girls, the researchers do not advise parents to control their child’s weight to delay puberty. Instead, parents should be more health-conscious.

“I don’t want to have a nation of patients with eating disorders. We need to figure out what are healthy weights for our kids. We want them to be comfortable with their bodies,” Biro said.

Currently in the U.S., one out of three kids is considered to be overweight or obese, according to the Centers For Disease Control and Prevention. Obese youth have an increased risk for bone and joint problems, sleep apnea, and social and psychological problems such as poor self-esteem.

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Source: Biro FM, Greenspan  LC, Galvez MP, et al. Onset of Breast Development in a Longitudinal Cohort. Pediatrics. 2013.