Final answer:
Hypertension is the most common factor associated with the increased incidence of placental abruption, even though smoking and other factors also contribute to the risk.
Step-by-step explanation:
The most common factor associated with the increased incidence of placental abruption is hypertension. Although factors such as advanced maternal age, multiparity, and smoking contribute to the risk of placental abruption, hypertension is notably a significant risk factor. It's important to note that smoking does have serious implications during pregnancy, leading to complications such as growth, preterm labor, and increased perinatal morbidity and mortality. Hence, quitting smoking is highly recommended for pregnant women to reduce the risk of such complications, including placental abruption.