What drives earlier menstruation in girls?

pre-teen girls outdoors

If Judy Blume’s classic book, Are You There God? It’s Me, Margaret, had been written in the 1890s, the title character – a girl anxiously awaiting her first period – would have been a senior in high school and not the sixth grader Blume depicts. Between the 1890s and the 1950s, the average age at menarche – the medical term for first menstruation – fell from 17 to 12.

The age at which most girls get their periods has fallen much more slowly since the ’50s, but it remains a topic of medical interest and concern – among other reasons because earlier menarche conveys increased risk of breast cancer.

Robert Hiatt, MD, PhD, studies social risk factors for breast cancer, and, as a PI of the Breast Cancer and the Environment Research Program, he saw an opportunity to cut through some of the methodological challenges to look at what influences a girl’s age at menarche. The research included interviews and physical examinations of more than 1,000 pre-pubescent girls, aged 7 to 8, from diverse ethnic backgrounds, allowing researchers to track in real time when the girls began puberty and when they started to menstruate.

Most believe that the dramatic biological shift of the first half of the 20th century was a result of factors associated with industrialization: As more girls received better nutrition, they tended to reach reproductive maturity at a younger age. Hiatt wondered how present-day girls’ socioeconomic status affects their age at puberty and menarche. How does obesity, which disproportionately affects poorer Americans, fit in? Do genetic differences account for the earlier menarche seen in Hispanic and Black girls, or is socio-economic status the driving factor?

After adjusting for the girls’ percentile on the body mass index (BMI) chart for their age, Hiatt and his colleagues found that lower socioeconomic status remained a significant predictor of earlier puberty and menarche in each of the four racial groups (Asian American, Black, Latina and white). When the researchers adjusted for both BMI percentile and race, the effect of socioeconomic status was too small to be statistically significant.

The hypothesis, Hiatt explained, is that in addition to making obesity more likely, lower socioeconomic status introduces adversity into girls’ lives. Evolutionary biology posits that adversity triggers what’s called the “thrifty gene hypothesis.” Basically, nature favors survival of the gene pool over that of the individual, so that when the body perceives a threat to survival, it sends a signal to hurry up and procreate.

Support for this hypothesis comes from earlier research, including Hiatt’s, finding that girls who grew up without a father present also reach puberty earlier, which is consistent with research on other social mammals.

“The age of menarche is decreasing, and in addition to what we know about BMI and race and ethnicity, it seems like socioeconomic status also has an effect,” Hiatt said. “It’s not as strong, but it’s there, and it needs to be explored further,” Hiatt said.

It remains for future work to disentangle socioeconomic status from another risk factor for cancer: chemical exposures. Research has shown that people in lower-income neighborhoods have higher exposures to industrial chemicals. But girls from more well-to-do families may have higher exposures to the endocrine-disrupting chemicals often found in household products and cosmetics, Hiatt said.

Photo: Melissa Askew