Around the world, puberty is starting earlier and earlier. Per US data, the average age of menarche — a girl’s first menstrual cycle — is roughly age 13 today; at the start of the 20th century, it was 16 to 17. Breast development, too, the first sign of puberty in girls, is starting one to four months earlier on average than it used to. Per Newsweek: “Just a generation ago, less than 5% of girls started puberty before the age of 8; today that percentage has more than doubled,” note Dr. Louise Greenspan and Dr. Julianna Deardorff in The New Puberty: How to Navigate Early Development in Today’s Girls.

Across the world, in China, the starting age of normal puberty in boys and girls is skewing younger, as well. Indian academics have only begun to collect such data, but some of the findings so far suggest its young people are also part of the trend. The question is: why?

To date, most scientists agree it’s a mix of many factors — some positive, some negative — associated with modern living. For instance, studies of immigrant communities and adopted refugee children show improved nutrition contributes to a younger start to normal puberty. Other research suggests toxic compounds, notably pesticides, and phthalates, which are found in plastics, can disrupt developing bodies’ hormonal systems and fat cells, affecting the age of puberty onset.

“The No. 1 factor that was pushing girls into puberty early was their body mass index,” Dr. Frank Biro, director of education and a professor in the Division of Adolescent Medicine at Cincinnati Children’s Hospital Medical Center, told Newsweek.

Indeed research in India is bearing this out. In separate studies of both boys and girls, the onset of puberty — characterized clinically as breast and pubic hair development for girls, and genital and pubic hair development in boys — occurred at a younger age among children considered overweight or obese. (In the study of boys, only early pubic hair development was associated with a higher BMI.)

But while these environmental factors combine to increase fat storage and turn children into adults (at least physiologically) sooner, the seeds of early puberty might be laid before kids are even born.

A new study of more than 15,000 girls and their mothers by Kaiser Permanente, a US health care provider, has linked maternal weight and hypoglycemia during pregnancy to early puberty in girls age 6 to 11.

“We know that maternal weight can influence childhood weight. What we are learning is that the in utero environment may also affect the timing of future pubertal development in offspring, which makes sense since human brains are developed in utero and the brain releases hormones affecting puberty,” said lead author Ai Kubo, PhD, a research scientist with the Kaiser Permanente Northern California Division of Research.

Researchers found that maternal obesity (a body mass index of 30 or more) and overweight (body mass index between 25 and 30) in mothers were associated with a 40% and 20%, respectively, greater chance of earlier breast development in girls. The study, published in the American Journal of Epidemiology, also found a 7-month difference in onset of breast development in daughters of obese versus underweight mothers.

For pubic hair development, similar associations between maternal obesity and earlier onset were found. However, the data suggest that the associations may differ by race and ethnicity. For instance, Asian girls with obese mothers were 50% more likely to experience earlier pubic hair growth than Asian girls with normal-weight mothers, while there were no associations among African-American girls.

The study also found a significant relationship between hyperglycemia (elevated blood sugar during pregnancy) in mothers and earlier breast development in daughters, but not in mothers with gestational diabetes.

“It’s possible that women with the diagnosis of gestational diabetes were more careful about weight and diet, which might have changed the amount of weight gain and offspring development patterns, but other studies need to replicate the finding to be able to conclude that there is an association,” Kubo noted.

Research is increasingly revealing the pre-pregnancy health of both parents and the fetal environment during pregnancy can affect kids’ development later in life. Earlier, The Swaddle reported on how fathers’ stress can affect sperm health and a baby’s future health, while maternal stress during pregnancy can affect brain development.

While early puberty seems less dire than inhibited brain development, it is cause for concern. The psychological fallout of appearing — and being treated as — years older than your mental age can be profound. Biro and others have also found girls who go through early puberty are more at-risk for breast and reproductive tract cancers long-term. As the starting age of puberty skews younger and younger, it means puberty is lasting longer and longer, and puberty is a particularly susceptible developmental window, when body tissue is vulnerable to social and environmental stressors. Much less research exists into early puberty in boys, a gap science needs to fill, and soon.

Regardless, there doesn’t seem to be much parents can do to prevent early puberty. Most experts acknowledge a new normal around puberty’s average starting age, which means cases of precocious puberty — a clinical term for extremely early puberty that begins before age 8 for girls and age 9 for boys — are on the rise, too, though still rare (the sensationalizing that characterizes reporting around these cases is unwarranted). The biggest preventative actions parents can take seem to be taking care of their own health, giving kids healthy, balanced meals, and minimizing (there’s no way to completely avoid) use of products that contain phthalates: use glass, stainless steel or silicon food containers; avoid plastics (especially plastic toys) with recycling codes 3 or 7; wash food well; and avoid scented products.