Smoking is the most preventable maternal factor for intrauterine growth restriction (IUGR) because it directly causes placental vasoconstriction and reduced oxygen delivery to the fetus. This leads to impaired fetal growth and development. Unlike maternal age, genetic factors, or multiple gestations, smoking is a modifiable behavior that can be eliminated through cessation programs, significantly reducing the risk of IUGR. Effective counseling and interventions targeting smoking cessation during pregnancy are crucial for improving fetal outcomes.

Reference: Williams Obstetrics, Cunningham et al., 26th Edition.