Researchers found that when pregnant women breathe in black carbon pollution -- created by the combustion of fossil fuels, such as in diesel-powered cars or the burning of coal -- harmful particles can make their way from the lungs to the placenta and may reach fetuses directly.

Dirty air has previously been linked to increased miscarriages, premature births and low birth weights among infants, as a result of the effects of pollution on the mother.

However, the placenta -- an organ that attaches itself to the womb during pregnancy, allowing oxygen and nutrients to pass from the mother's blood supply to the fetus through the umbilical cord -- was previously thought to be an "impenetrable barrier."

A study last year was the first to suggest this wasn't the case, after pollutants were found in the placentas of five pregnant women in the United Kingdom.

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