For many years, public health agencies considered 10 micrograms per deciliter (μg/dL) of lead in maternal blood to be an acceptable upper limit. However, earlier research raised concerns that this threshold may not have been protective enough for fetal brain development.

A study published in Environmental Health Perspectives (June 2004) examined maternal blood lead levels throughout pregnancy and tracked their children’s intelligence scores up to age 10. Lead was measured in the mothers at 12, 20, 28, and 36 weeks of pregnancy and again at birth. Lead levels in the children were tested twice yearly until age five, and annually from six to ten years old. Children’s IQ scores were also assessed annually between ages six and ten.

What the researchers found

  • Higher maternal lead levels during the third trimester were linked with lower IQ scores in children between ages 6 and 10.

  • The third trimester is a period of rapid brain development, which may explain the heightened vulnerability.

  • The relationship was dose-dependent—as maternal blood lead increased, IQ declined.

  • Notably, half of the total drop in IQ occurred at levels well below 10 μg/dL, suggesting that even a few micrograms per deciliter can have meaningful effects.

This study was one of several that helped shift scientific understanding toward the now widely accepted view that there is no known safe level of lead exposure, especially during pregnancy.

Is the old “10 μg/dL limit” still considered acceptable today?

No. Major health organizations have lowered their thresholds significantly:

  • The CDC currently uses a blood lead reference level of 3.5 μg/dL for children (as of 2021) to identify exposure that requires follow-up.

  • Many researchers and public health agencies now state that any detectable lead can be a concern during pregnancy, because the fetus is especially vulnerable.