A developmental psychologist is reviewing the case file of a child diagnosed with a disorder characterized by significant inattention and hyperactivity. The psychologist notes that the child's symptoms are particularly severe. Based on established research findings, which piece of information from the mother's prenatal history would be most directly associated with both the presence and the heightened severity of the child's condition?

A researcher conducts a longitudinal study and finds that among children diagnosed with a disorder characterized by inattention and hyperactivity, those with the most severe symptoms are significantly more likely to have had mothers who smoked heavily during pregnancy. Based solely on this correlational finding, what is the most scientifically sound conclusion?