After decades of research, the puzzle surrounding why some children develop autism spectrum disorder (ASD) continues to shake off easy answers, as no one single factor seems to be responsible. Currently, most scientists agree that a combination of genetic and environmental influences lead to the condition.

Exposure during pregnancy or early childhood to heavy metals such as mercury and lead has been linked to autism for decades, and the infamous, now-disproven link between vaccines and ASD stems in part from a theory that a mercury-containing preservative in some vaccines acts as a neurotoxin. Yet beside for avoiding obviously dangerous levels of mercury, the medical community’s advice on the safety of exposure to low levels of mercury has been conflicting, thanks to the lack of solid evidence from large, prospective human studies.

This lack of consensus has been particularly challenging for pregnant women who enjoy a fish-heavy diet or desire to eat fish because they are rich sources of the omega fatty acids known to benefit fetal health.  

Now, insights from a nearly 20-year-long ongoing study by the University of Bristol appear to (finally) put the issue to rest. After analyzing data from more than 4,000 pregnancies, a team of authors found no association between the levels of mercury in the mother’s bloodstream and the child’s later diagnosis or absence of either ASD or individual autistic traits up to age 11.

"Our findings further endorse the safety of eating fish during pregnancy,” stated Professor Jean Golding, lead author of the investigation published in Molecular Autism. “Importantly we've found no evidence at all to support claims that mercury is involved in the development of autism or autistic traits.”

Overall, Golding and her colleagues enrolled more than 14,000 women living in the Avon area of the UK who were pregnant between 1991 and 1992 in order to study long-term effects of lifestyle and environment on childhood development. The current study included 4,484 women whose trace mercury levels were objectively measured through blood samples, and whose possible exposures were elucidated through questionnaires on fish consumption and the presence of mercury alloy dental fillings.

Of the 177 pregnancies that led to an ASD child, only 45 had any detectable mercury, and the average level was nearly identical to that of 3,840 pregnancies that led to neurologically typical children.

Quite notably, poor social cognition – a hallmark ASD trait – was actually more common in the children of women who did not eat fish during pregnancy, after adjusting for other risk factors.

In seeming preparation for blowback by impossible-to-please skeptics, the authors conclude by highlighting the strengths of their study.

“This is the largest prospective population study to date to address this question. It is the only study to compare total blood mercury levels in the first half of pregnancy among offspring with autism or high scores on autistic traits. It is also the only study to determine whether the exposures known to result in increased mercury levels were associated with autistic outcomes in the offspring.”