Final answer:
The classic symptom associated with placental abruption is vaginal bleeding, and it is considered an obstetric emergency.
Step-by-step explanation:
The classic symptom associated with placental abruption is vaginal bleeding. This condition can also present with other symptoms, including abdominal pain or back pain, uterine contractions, and in severe cases, an absence of fetal movements. Placental abruption is a serious pregnancy complication where the placenta detaches from the uterine wall before delivery, which can lead to significant hemorrhage and is an obstetric emergency requiring immediate medical attention.