**Core Concept**
Anencephaly is a severe congenital anomaly characterized by the absence of a major portion of the brain, skull, and scalp. Early detection is crucial for counseling and management of the pregnancy.

**Why the Correct Answer is Right**
The ideal timing to detect anencephaly is between 16 and 20 weeks of gestation, using ultrasound. During this period, the fetal cranial vault is still open, and the absence of the cranial bones can be visualized. The detection rate of anencephaly increases with gestational age, and it is recommended to perform a detailed ultrasound scan between 16 and 20 weeks. This timing allows for accurate diagnosis and counseling of the parents.

**Why Each Wrong Option is Incorrect**
**Option A:** Early detection is not recommended before 16 weeks, as the fetal cranial vault is still closed, and the absence of the cranial bones may not be visible.
**Option B:** Detection after 20 weeks is less accurate, as the cranial bones may start to close, and the absence of the cranial vault may not be visible.
**Option C:** Anencephaly can be detected earlier using other imaging modalities, such as MRI or CT scan, but ultrasound is the preferred method due to its non-invasive nature and lack of radiation exposure.

**Clinical Pearl / High-Yield Fact**
Anencephaly is a significant indicator of a higher risk of other congenital anomalies, and a detailed ultrasound scan is recommended to detect any associated abnormalities.

**Correct Answer: C. 20 weeks.