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An alarming new study found dozens of "mystery chemicals" inside the bodies of pregnant women, leaving scientists both concerned and confused.

Of the 109 unusual, human-made chemicals identified in the study, 55 had never been found inside the human body before, according to research published last week in the journal Environmental Science & Technology. Another 42 were complete mysteries — both in terms of where they came from and what they even are.

The discovery highlights the lack of regulatory oversight when it comes to the chemical industry using potentially hazardous chemicals without disclosing what they are, the University of California, San Francisco scientists behind the study said in a press release.

"It's very concerning that we are unable to identify the uses or sources of so many of these chemicals," said lead study author and UCSF reproductive sciences expert Tracey Woodruff. "[The Environmental Protection Agency (EPA)] must do a better job of requiring the chemical industry to standardize its reporting of chemical compounds and uses. And they need to use their authority to ensure that we have adequate information to evaluate potential health harms and remove chemicals from the market that pose a risk."

When it comes to the 55 known chemicals, knowing what they are didn't provide much reassurance. Some are used as pesticides, packaging materials and other plastics, and most have so little information about them available that the UCSF researchers couldn't figure out where they came from how the patients ingested them. That's concerning news, especially since compounds can readily be spread from mother to child, posing a threat to both.

"It is alarming that we keep seeing certain chemicals travel from pregnant women to their children, which means these chemicals can be with us for generations," Woodruff, who used to work for the EPA, added.