Final answer:
The nurse should emphasize that smoking is the most common and harmful modifiable risk factor during pregnancy, significantly increasing the risk of various prenatal and postnatal complications.
Step-by-step explanation:
When teaching health promotion during a prenatal class, the nurse should list smoking as the most common and harmful modifiable risk factor during pregnancy. Smoking during pregnancy exposes the fetus to various risks through competitive binding of carbon monoxide with hemoglobin as well as other components in cigarettes that lead to adverse biological effects. It is associated with a plethora of prenatal and postnatal complications including growth reta_rdation, preterm labor, abruption of the placenta, spontaneous abortion, and sudden infant death syndrome (SIDS).