Why Are Girls Starting Puberty Earlier?
It is common knowledge that most girls get their periods at around the age of 12. What fewer people know is that over the years, girls have been getting their periods earlier. According to the National Center for Health Statistics (NCHS), the average age of a person's first period declined from 12.1 years old in 1995 to 11.9 in 2013-2017. This may not seem too extreme, but it is part of a larger trend.
Based on bone length analyses, it seems that Palaeolithic girls may have experienced menarche (their first period) between the ages of 7 and 13. After people settled into agricultural societies, however, girls often didn't get their periods until their mid-to-late teens. In places where sanitation and nutrition improved, people started getting their periods earlier around the 18th century, but even in 1850, the average age of menarche in northern Europe was around 16. This dipped below 15 by the beginning of the 20th century and now hovers around the age of 12. While this decline in menarcheal age has slowed down since the 1960s, girls have started puberty earlier, with some girls even starting to develop breasts and pubic hair around the age of 6 or 7 (per Nature).
Why girls are going through puberty earlier
It is good that girls no longer get their first periods at 16, as that was largely a consequence of malnutrition and poor living conditions. That being said, early puberty isn't great news either. It is associated with a number of health risks, such as cancer and diabetes, later in life. Dr. Hector Chapa tells Texas A&M Today that part of this may be because of early and prolonged exposure to estrogen. Girls who start puberty early are also more likely to develop depression.
Nobody knows exactly why people are starting puberty earlier, but there are a number of theories. Jessa Gamble's 2018 article in Nature notes that, "In animals, more access to food correlates with earlier puberty," and that early puberty may go hand-in-hand with an increase in body mass index (BMI). Indeed, the prevalence of obesity among children has doubled in the past few decades (per Global Pediatric Health), and a 2003 study published in BMJ Pediatrics found that every standard deviation increase in BMI doubled a child's risk of early menarche.
Gamble proposes that artificial light could play a role, as children exposed to more natural light tend to start puberty earlier. Another potential culprit is the range of endocrine-disrupting chemicals in food and in the environment. The increase in single-parent households may also play a role, as girls who receive more care from their fathers tend to start puberty later.