## Core Concept
A Level II ultrasound scan, also known as a targeted or detailed ultrasound, is performed during pregnancy to evaluate fetal anatomy in greater detail than a standard ultrasound. This scan is particularly crucial for detecting potential fetal anomalies.

## Why the Correct Answer is Right
The correct answer, , corresponds to the gestational age of 18-20 weeks, which is the optimal period for performing a Level II ultrasound scan. At this stage, the fetus is developed enough for a detailed examination of its anatomy, but still early enough for potential interventions if anomalies are detected. This scan focuses on assessing fetal structures such as the brain, spine, heart, limbs, and other organs to identify any congenital anomalies.

## Why Each Wrong Option is Incorrect
* **Option A:** This gestational age is too early for a detailed Level II scan as the fetus may not be developed enough for a comprehensive assessment of its anatomy.
* **Option B:** Although some basic ultrasound assessments can be done at this stage, it's generally considered too late for the primary Level II scan aimed at detecting congenital anomalies.
* **Option C:** This option might seem plausible but is not the standard timeframe for a Level II scan.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the 18-20 week scan is critical for detecting congenital anomalies and for fetal anatomy survey. It's also a period when certain fetal conditions can be better evaluated due to the size and development of the fetus.

**Correct Answer: D. 18-20 weeks.**