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Study: Prenatal Exposure to Mercury Not Associated with Autism

Study finds no link between autism and mercury levels in mom or newborn; still prudent to limit intake of certain fish during pregnancy
July 30, 2014

A new study finds no association between autism and mercury levels in blood samples from pregnant women or their newborns. But the researchers caution that pregnant women should continue to heed long-standing advice to limit their consumption of fish that contain high levels of this potentially toxic pollutant. 

The findings appear in the journal Environmental Research.

Exposure to high levels of methyl-mercury during pregnancy is a long-standing concern. Previous research has linked it to a number of disorders of brain development, including intellectual disability and seizures. Methyl-mercury pollution is widespread in oceans and lakes. And this form of mercury tends to accumulate in certain fish such as halibut and tuna – as well as in people who eat them.

Methyl-mercury is distinct from ethyl-mercury, a form of mercury that the body eliminates more quickly. Research has consistently found no association between autism and ethyl-mercury, which is in the preservative thimerosal. Still, as a precautionary measure, the U.S. Food and Drug administration ordered that childhood vaccines no longer be made with thimerosal as of 1999, with the exception of flu shots. (Individuals can request flu shots without the preservative.)

High mercury levels found 
In the new study, members of the California Department of Public Health’s Early Markers for Autism project looked at the possible role that prenatal mercury exposure might play in determining risk for autism.

Using anonymous patient records from California’s public health system, they identified 84 children with autism, 49 children with intellectual disability or developmental delay and 150 developmentally typical children. They then looked at total mercury levels in stored, newborn blood samples from these children. They also analyzed blood samples from their mothers that were taken midway through their pregnancies.

A third of the newborn blood samples and 15 percent of the blood samples from pregnant mothers contained levels of mercury higher than is considered safe by the U.S. Environmental Protection Agency. This provides further evidence that mercury pollution remains a serious public health concern.

No difference in autism risk
However, the investigators found no overall difference in blood mercury levels between any of the three groups of children or their mothers. In other words, the children who developed autism, intellectual disability or developmental delay were no more likely to have had high mercury levels at birth than were the typically developing children. Nor were they more likely to be born to mothers who had high levels of mercury in their blood during pregnancy.   

The investigators call for larger studies to confirm their findings and explore additional risk factors. For example, research is needed to investigate how a person’s genetic makeup may influence the relationship between mercury levels and autism risk.

“Pregnant women should do all they can to reduce mercury exposure during pregnancy, especially through diet,” says Alycia Halladay, Autism Speaks senior director for environmental and clinical sciences. “However, given these results, high mercury levels during pregnancy do not appear to be responsible for an autism diagnosis.”

Read Dr. Halladay’s broader advice about avoiding toxic exposures during pregnancy here.

For a consumer guide to mercury levels in fish, click here.