**Core Concept**
Asymmetric growth retardation (AGR) in a fetus refers to a pattern of growth restriction where the head size is relatively spared compared to the rest of the body. This occurs due to placental insufficiency, which affects the umbilical artery blood flow and oxygen delivery to the fetus.

**Why the Correct Answer is Right**
Placenta previa is a condition where the placenta partially or completely covers the internal cervical os. This can lead to placental insufficiency, which in turn causes asymmetric growth retardation in the fetus. The placenta's abnormal implantation site can compromise the umbilical artery blood flow, resulting in inadequate oxygen and nutrient delivery to the fetus. As a result, the fetal body grows at a slower rate compared to the head, leading to asymmetric growth retardation.

**Why Each Wrong Option is Incorrect**
**Option A:** Anencephaly is a congenital anomaly characterized by the absence of a major portion of the brain and skull. It is not directly associated with asymmetric growth retardation.

**Option B:** Trisomy 18, also known as Edwards syndrome, is a chromosomal disorder that affects fetal growth and development. However, it is typically associated with symmetric growth restriction rather than asymmetric growth retardation.

**Option C:** Congenital rubella syndrome can cause various fetal anomalies, including growth restriction. However, it is not specifically associated with asymmetric growth retardation.

**Clinical Pearl / High-Yield Fact**
Asymmetric growth retardation is often seen in cases of placental insufficiency, which can be caused by conditions such as placenta previa, placenta accreta, or uteroplacental vascular insufficiency.

**✓ Correct Answer: D. Placenta previa. Placenta previa is associated with asymmetric growth retardation in a fetus due to placental insufficiency and compromised umbilical artery blood flow.**