Particles of black carbon - or soot - can cross the placenta, a study has found.
The Nature Communications research is the first direct evidence the particles can get into the part of the placenta that feeds the developing fetus.
It could be the first step to explaining why high pollution is linked to an increased risk of miscarriage, premature birth and low birth weights.
Experts said women could take measures such as avoiding busy roads.
But they warned tackling air pollution could be achieved at a "policy level" only.
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Original article from the BBC