Researchers found that among more than 1,000 pregnant women they followed, those who used certain hair products -- dyes, bleaches, relaxers or mousse -- had lower levels of several hormones, including estrogen and progesterone.
That's a concern because during pregnancy, levels of those hormones should rise, said lead researcher Zorimar Rivera-Nunez, an assistant professor at Rutgers School of Public Health in Piscataway, N.J.
Past research, she noted, has linked disruptions in pregnancy hormones to an increased risk of problems such as impaired fetal growth, preterm birth and low birth weight.
How would hair care fit in? Personal care products, including lotions, cleansers, makeup, shampoo and nail polish, often contain many chemicals. And they include so-called "endocrine disruptors" -- chemicals that can interact with the body's hormonal system.
Endocrine disruptors are everywhere, and people can be exposed through food, water or even the air they breathe, according to the Endocrine Society. When it comes to personal care products, some of the common hormone-disrupting chemicals include parabens, phthalates, bisphenol-A and toxic metals.
Researchers are still trying to figure out how exposure can affect human health, Rivera-Nunez said. It's complicated, in part, because people are habitually exposed to numerous chemicals.
But studies have found, for example, that when expectant women have high levels of certain endocrine disruptors in their bodies during pregnancy, their offspring are more likely to become overweight, or go through early puberty.
Similarly, there is evidence tying personal care products, specifically, to health risks.
A U.S. government study found that women who frequently used chemical hair straighteners had a higher breast cancer risk than nonusers. Hair dyes were also tied to an increased risk of the disease, particularly among Black women.
As for pregnancy, one recent study of pregnant women in China found that those who frequently used makeup or skin care products were more likely to have a baby who was small for gestational age -- a sign of growth restriction in the womb.
The new study "fits in well" with that overall body of research, said Alexis Temkin, a toxicologist with the nonprofit Environmental Working Group in Washington, D.C.
It links hair product use to hormonal differences that are consistent with some of the health effects that have been tied to such products, according to Temkin.
The findings -- published in the journal Environmental Research -- are based on 1,070 pregnant women in Puerto Rico who made up to three study visits over the course of their pregnancy. They completed questionnaires on personal product use and gave blood samples to have their hormone levels measured.
Overall, levels of estrogen, progesterone and testosterone were lower among women who reported using "other" hair products, versus nonusers. That category included dyes, straighteners, bleaches and mousse, but not shampoo, conditioner, hair spray or hair gel.
It's not clear, according to Rivera-Nunez, whether women who use those hair products might be exposed to particular chemicals that are problematic, or have a higher level of exposure to endocrine disruptors.
Beyond that, there are many factors that might sway pregnancy hormones. The researchers factored in the variables that they could -- such as women's pre-pregnancy body weight, income and education levels, as well as their smoking and drinking history.
But it's not possible to account for everything, Rivera-Nunez said.
For now, she recommended that women who are pregnant or planning a pregnancy read labels and be aware of what they are putting on their bodies. At the same, she acknowledged that those labels are not necessarily consumer-friendly.
"The lack of good labeling is a problem," Rivera-Nunez said.
Temkin advised looking out for the word "fragrance" -- a harmless-sounding term that actually includes a broad range of undisclosed chemicals, some of which may be endocrine disruptors.
The Environmental Working Group has more on personal care products' ingredients.
SOURCES: Zorimar Rivera-Nunez, PhD, MS, assistant professor, biostatistics and epidemiology, Rutgers School of Public Health, Piscataway, N.J.; Alexis Temkin, PhD, toxicologist, Environmental Working Group, Washington, D.C.; Environmental Research, Nov. 17, 2021, online