Final answer:
The nurse should identify smoking as the most common and harmful modifiable risk factor during pregnancy, emphasizing its associated risks and the importance of cessation for prenatal care.
Step-by-step explanation:
During a prenatal class, when providing health promotion teaching, the nurse should list smoking as the most common, yet harmful, modifiable risk factor during pregnancy.
Smoking during pregnancy can lead to a wide range of adverse outcomes including premature birth, low birth weight, and increased risk of sudden infant death syndrome. Therefore, cessation of smoking is a critical component of prenatal care and health promotion for expectant mothers.