**Core Concept**
The diagnosis of fetal aneuploidy relies on identifying specific features that indicate an abnormal number of chromosomes in the fetus. Fetal aneuploidy occurs when there is an extra or missing chromosome, which can lead to various congenital anomalies and developmental disabilities.

**Why the Correct Answer is Right**
Nuchal translucency (NT) is a critical sonographic marker used to assess the risk of fetal aneuploidy. It measures the thickness of the fluid-filled space at the back of the fetal neck, which is usually around 2.5 mm at 11-14 weeks of gestation. An NT thickness greater than 3.5 mm is associated with an increased risk of aneuploidy, particularly Down syndrome. This is because the excess fluid in the NT space can be a sign of abnormal fetal development.

**Why Each Wrong Option is Incorrect**
**Option A:** Maternal age is a significant risk factor for aneuploidy, but it is not a direct diagnostic feature. While older maternal age is associated with an increased risk of Down syndrome, it is not a specific marker for aneuploidy.
**Option B:** Fetal heart rate abnormalities can be associated with aneuploidy, but they are not as specific or sensitive as nuchal translucency. Fetal heart rate abnormalities can be caused by various factors, including aneuploidy, but they are not a definitive diagnostic feature.
**Option C:** Fetal size and growth restriction can be associated with aneuploidy, but they are not as specific or sensitive as nuchal translucency. Fetal growth restriction can be caused by various factors, including aneuploidy, but it is not a definitive diagnostic feature.

**Clinical Pearl / High-Yield Fact**
The 1 in 1,000 rule is a useful mnemonic to remember the risk of Down syndrome associated with nuchal translucency. For every 1 mm increase in NT thickness above 3.5 mm, the risk of Down syndrome increases by approximately 1 in 1,000.

**Correct Answer:** C. Nuchal translucency.