Why are girls entering puberty earlier?

Reproductive Health

Girls are getting their first period at younger and younger ages, especially girls of color and of low socioeconomic status.  Potential reasons include both higher BMI and stress, but it’s still not fully understood.  This change is important, as earlier sexual maturation brings with it concerns for both physical and social health.

recent study of over 70,000 subjects identified that on average, girls* are entering puberty earlier than ever.  The average age at first menstrual period, called menarche, decreased from 12.5 years for those born between 1950 and 1965 to 11.9 years for those born between 2000 and 2005.  The trend was especially pronounced in women of color (mean age 11.6) and of low socioeconomic status (mean age 11.8), but the change occurred across the board.  Additionally, the percentages of girls getting their periods early about doubled:  menarche before age 11 rose from 8.6% to 15.5%, and before age 9 went from 0.6% to 1.4%.

The exact reasons for this decrease in age are complex and not fully understood.  Worldwide, age at menarche has been decreasing for more than 100 years.  For most of that time, however, it was an indicator of success.  Improvements in nutrition, hygiene, and medicine resulted in a healthier population, which meant that puberty was no longer delayed as it had been previously.  At some point in the last 50 years, however, we seem to have crossed a line where this change is no longer an indicator of improved health but rather a potential sign that things are amiss.  There are several hypotheses for why this is happening.

🔸 Higher BMI and body fat.  The link between increased BMI or body fat and earlier onset of puberty and menarche has been well demonstrated.  In the new study, the authors estimated that about 46% of the decrease in age at menarche was likely due to increased BMI in the study subjects over time.  So, this is absolutely a piece of the puzzle, but it is not the only factor.

🔸 Stress.  Exposure to stress early in life, such as through Adverse Childhood Experiences (ACEs) and low socio-economic status, is known to be associated with earlier puberty.  Exactly why this occurs has not yet been worked out.  In the recent study, over time the proportion of subjects reporting low socioeconomic status increased by 10%, which is similar to the changes in US household economic status over the same period of time.  So higher economic stress could be contributing to the population changes we’re seeing, but it’s not the only reason, as those in the high SES group also had a drop in age at menarche.

Higher stress is also hypothesized to be the cause of the lower ages of menarche seen in girls of color, another example of what is called “weathering.” Exposure to discrimination, harassment, and microaggressions in many ways leads to premature aging and worse health outcomes. (FYI – there have been found to be no genetic differences in people of color that account for earlier age at puberty.)

🔸 Endocrine Disrupting Compounds (EDCs).  Many of the products we currently use, especially plastics and pesticides, can disrupt our internal hormone (endocrine) system in a variety of ways.  Some studies of acute high exposure to certain chemicals show a risk of earlier menarche, but studies of general populations have, by and large, not found any consistent link between exposure to EDCs and earlier menarche. National Library of Medicine: Endocrine-Disrupting Chemicals and Early Puberty in Girls. Oxford Academic: Prenatal and postnatal exposures to endocrine disrupting chemicals and timing of pubertal onset in girls and boys: a systematic review and meta-analysis

🔸 The Pandemic. Other recent studies have reported a significant increase in diagnoses of “precocious puberty,” or puberty before age 8, in girls that occurred during the COVID-19 pandemic. Potential reasons cited include both stress and changes in behaviors, such as activity level, food choices and eating pattern, and screen time. We don’t yet know what impact the pandemic has had on menstrual onset for children in general.

Earlier menarche is associated with physical and mental health risks.  Rates of depression, anxiety, substance abuse, eating disorders, low self esteem, and negative body image are higher.  Bullying is more common.  Lifetime risk of heart disease and breast and endometrial cancer is also higher.  Troublingly, girls who mature early get treated differently by adults, especially adult men.  Lastly, most of our educational systems are also not prepared for earlier onset of puberty in girls.

Overall, we seem to have crossed a line where earlier menarche is no longer a positive indicator.  Higher BMI and body fat and higher levels of stress may be big drivers of this earlier puberty in girls, but much is still to be learned.  In the meantime, we need to adapt our social systems to support children with earlier menarche and to safeguard their physical and mental health.

*The terms “girls” and “women” are used in this post, as those were the characterizations of subjects for the studies mentioned.  Other people menstruate, too, and we can anticipate that earlier puberty in transgender children would create additional significant challenges for that population.

The main study

About the Weathering Effect

STAT article about mental health risks of early puberty in girls

No strong evidence for EDCs as drivers of earlier puberty in general population:

National Library of Medicine: Endocrine-Disrupting Chemicals and Early Puberty in Girls

Oxford Academic: Prenatal and postnatal exposures to endocrine disrupting chemicals and timing of pubertal onset in girls and boys: a systematic review and meta-analysis