**Core Concept**
Placenta previa is a condition where the placenta partially or completely covers the internal os of the uterus, posing a risk of severe bleeding during pregnancy. Conservative management is typically preferred in cases of placenta previa, but there are certain situations where it is contraindicated.

**Why the Correct Answer is Right**
In cases of placenta previa, conservative management is contraindicated in situations where there is active bleeding that is not controlled with medical management, such as vaginal bleeding that is heavy or persistent. This is because conservative management relies on the uterus to contract and stop bleeding, but if the bleeding is too severe, it may not be possible to control it without surgical intervention. In such cases, **emergency cesarean delivery** may be necessary to save the mother's life.

**Why Each Wrong Option is Incorrect**
**Option A:** Active bleeding that is not controlled with medical management is a contraindication for conservative management, making this option correct.
**Option B:** **Painless vaginal bleeding** in the second or third trimester is a common presentation of placenta previa, but it is not a contraindication for conservative management.
**Option C:** **Previous uterine surgery** may increase the risk of placenta previa, but it is not a contraindication for conservative management.
**Option D:** **Previous uterine scar** from a cesarean delivery or other uterine surgery may increase the risk of placenta previa, but it is not a contraindication for conservative management.

**Clinical Pearl / High-Yield Fact**
In cases of suspected placenta previa, a **pelvic ultrasound** is often used to confirm the diagnosis and assess the extent of placental coverage of the internal os. This is an important diagnostic tool that can help guide management and avoid unnecessary surgical interventions.

**Correct Answer: A.**