Ozone (the major component of smog) is formed through reactions between nitrogen oxides and volatile organic compounds (which are found in auto emissions, among other sources) in heatand sunlight. 
Ozone is a highly reactive compound that damages tissue, reduces lung function, and sensitizes the lungs to other irritants.
For example, exposure to ozone during exercise reduces lung functioning in adults and causes symptoms such as chest pain,
coughing, and pulmonary congestion. 
It is not clear why ozone would affect the fetus, though like PM10 it might indirectly affect the infant by compromising the mother’s health.