Girls exposed to chemicals commonly found in makeup and other personal care products before birth hit puberty earlier than their non-exposed peers, according to new research.

A study by the University of California has found that daughters of mothers who had higher levels of diethyl phthalate and triclosan in their bodies during pregnancy experienced puberty at younger ages.

Diethyl phthalate is often used as a stabiliser in cosmetics and fragrances while triclosan is an antimicrobial agent used in hand soaps, sanitisers and toothpastes.

The study recruited pregnant women in 1999 and 2000 and then followed their 338 children from birth to adolescence.

The researchers measured concentrations of phthalates, parabens and phenols in urine samples taken from the mothers during pregnancy, and from the children at the age of nine. They then followed the growth of the 159 boys and 179 girls from nine to 13 to track the timing of developmental milestones.

More than 90 percent of the urine samples of the mothers and children showed detectable concentrations of all three classes of chemicals, with the exception of triclosan which was present in around 70 percent of samples.

The researchers found that every time the concentrations of diethyl phthalate and triclosan in the mother’s urine doubled, the timing of developmental milestones such as the appearance of pubic hair and the start of menstruation in their daughters began approximately one month earlier.

Girls who had higher concentrations of parabens in their urine at nine years of age also experienced puberty at younger ages. However, it is unclear if the chemicals were causing the shift, or if girls who reached puberty earlier were more likely to start using personal care products at younger ages.

Kim Harley, an associate adjunct professor at the University of California’s School of Public Health told ScienceDaily that over the past 20 years studies have shown that girls have been experiencing puberty at progressively younger ages – and this is troubling news, as earlier age at puberty has been linked with increased risk of breast and ovarian cancer as well as mental illness.

“We know that some of the things we put on our bodies are getting into our bodies, either because they pass through the skin or we breathe them in or we inadvertently ingest them,” she said. “We need to know how these chemicals are affecting our health.”

She said that many researchers suspect that many chemicals in personal care products can interfere with natural hormones in our bodies, and studies have shown that exposure to these chemicals can alter reproductive development in rats but few studies have looked at how these chemicals might affect the growth of human children.

“We wanted to know what effect exposure to these chemicals has during certain critical windows of development, which include before birth and during puberty.

“While more research is needed, people should be aware that there are chemicals in personal care products that may be disrupting the hormones in our bodies.”

The study did not find any link between the chemicals and early puberty in the boys.