Arsenic in Pregnancy Linked to Lower IQ in Kids at Age 5, Study Finds

What a New Hampshire Cohort Found When It Tracked Six Metals Through Pregnancy and Into Age Five

Children whose mothers carried higher arsenic exposure around conception and during early pregnancy scored measurably lower on IQ tests at age 5 — a decline of roughly 0.16 standard deviations on full-scale IQ, according to new data from the New Hampshire Birth Cohort Study. The study, published in Environmental Health Perspectives by researchers at Dartmouth’s Geisel School of Medicine and Harvard’s T.H. Chan School of Public Health, also found that timing matters: the same metal can help or harm depending on when a child encounters it.

Arsenic Across Three Exposure Windows Predicted Lower IQ at Five

Led by Lorea Notario Barandiaran and colleagues including David Bellinger and Susan Korrick, the team followed 278 pregnancy–offspring pairs for roughly five years. They measured six elements — arsenic, copper, manganese, lead, selenium, and zinc — in toenail clippings collected from pregnant participants at 24–28 weeks of gestation (reflecting periconception and early pregnancy), at six weeks postpartum (reflecting mid-pregnancy), and from the infants themselves at six weeks of age (reflecting early infancy).

Toenails act like a slow-release archive: they capture circulating metal levels over the weeks before the clipping, giving researchers a biological record of exposure timing that a single blood draw cannot provide. Cognitive development was assessed with the Wechsler Preschool and Primary Scale of Intelligence (WPPSI-IV), which yields a full-scale IQ plus five composite indices spanning verbal reasoning, visual-spatial skills, working memory, and processing speed.

Arsenic stood out. Higher toenail arsenic at every one of the three time points predicted lower cognitive scores five years later. The strongest estimate for full-scale IQ was a 0.16 standard deviation decrease (95% credible interval: −0.24 to −0.08) per unit increase in log-transformed arsenic. That is not a catastrophic loss for an individual child, but across a population it shifts the distribution — and it appeared at exposure levels typical of a US cohort relying partly on private wells, a known arsenic source in New England.

Why Timing Matters More Than the Metal Itself

Biologically, the findings track with what we know about the developing brain. Arsenic crosses the placenta and interferes with folate-mediated methylation, DNA repair, and mitochondrial function — processes the fetal brain depends on during rapid synapse formation. It also disrupts glucocorticoid signaling, which shapes the stress-response circuitry being wired in the third trimester.

Manganese and copper showed the opposite pattern — but only during periconception and early pregnancy. Both are essential trace elements: manganese supports antioxidant defenses and bone formation, while copper is required for cytochrome c oxidase, the enzyme that powers neurons’ energy production. Adequate levels during organogenesis appear to support later cognition, consistent with broader evidence that nutrition in the first 1,000 days shapes brain development. The same metals can turn neurotoxic at high doses or later windows, which is why the timing result is scientifically important rather than contradictory.

The researchers also detected sex-specific effects, with manganese showing particularly divergent associations between boys and girls — a reminder that “average” findings can conceal different vulnerabilities. Lead in this cohort was generally low, which limited statistical power to detect its effects, a limitation the authors acknowledge; larger studies, including work on lead-contaminated water in child care centers, continue to find cognitive and behavioral harms from lead at low levels.

What This Means for Parents: Practical Steps That Reduce Exposure

The single most actionable finding concerns drinking water. Roughly 40 million Americans use private wells, which are unregulated and frequently exceed arsenic standards in New England and the upper Midwest. Testing well water costs $15–$50 through state laboratories; reverse-osmosis filters remove both arsenic and lead effectively, while standard pitcher filters do not.

Beyond water testing, the evidence supports straightforward measures: use cold, fully flushed tap water for infant formula, wash and peel root vegetables grown in soils with known metal contamination, and check for lead paint hazards in homes built before 1978. Nutrition matters too — adequate iron and calcium reduce gastrointestinal lead absorption, and nutrients like DHA support the neural circuitry that environmental toxins can perturb. For more on that connection, see what science really shows about DHA and child brain development.

Frequently Asked Questions

How does lead and arsenic exposure actually damage a child’s brain?

Both metals interfere with fundamental processes: arsenic disrupts DNA methylation and mitochondrial energy production in neurons, while lead mimics calcium, crossing the blood-brain barrier and derailing synapse formation, neurotransmitter release, and myelination during sensitive developmental windows.

Can a child recover cognitively after lead or arsenic exposure is reduced?

Partial recovery is possible, especially with early removal of exposure plus adequate nutrition, but some deficits can persist — which is why prevention during pregnancy and infancy matters far more than remediation later.

Are manganese and copper supplements good for my baby’s brain?

No — these metals helped at normal dietary levels during early pregnancy in the study, but excess amounts are neurotoxic. A balanced diet meets needs; supplements should only follow a diagnosed deficiency.

Do I need to worry if I don’t have a private well?

Municipal water is regulated for arsenic and lead, though older plumbing can still leach lead. If you’re on a well, testing is the clearest step — New England and parts of the upper Midwest have the highest arsenic prevalence.

One cohort of 278 families can’t settle every question, and these observational associations don’t prove causation on their own. But the pattern — arsenic harmful across every window, essential metals helpful in early windows, and timing as the deciding factor — aligns with a growing body of evidence that a child’s earliest environment is written, element by element, into their cognitive future.

Sources:
https://pubmed.ncbi.nlm.nih.gov/42148044/
https://pubmed.ncbi.nlm.nih.gov/42127906/
https://pubmed.ncbi.nlm.nih.gov/42022188/
https://pubmed.ncbi.nlm.nih.gov/42010680/
https://pubmed.ncbi.nlm.nih.gov/41849337/