## **Core Concept**
Nuchal translucency (NT) measurement is a critical component of first-trimester screening for aneuploidy, particularly for Down syndrome. It involves assessing the thickness of the fluid-filled space at the back of the fetus's neck. This measurement is taken via ultrasound and is used in conjunction with other markers and maternal age to estimate the risk of chromosomal abnormalities.

## **Why the Correct Answer is Right**
The correct gestational age for measuring nuchal translucency is between 11 weeks and 14 weeks of gestation. This period is chosen because it corresponds to a specific range of fetal crown-rump length (CRL), typically between 45 mm and 84 mm. During this time, the nuchal translucency thickness can be accurately measured, and its correlation with aneuploidy risk is well established. Accurate dating of the pregnancy and proper ultrasound technique are crucial for a reliable measurement.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Measurements taken outside the specified range (e.g., before 11 weeks or after 14 weeks) may not accurately predict the risk of aneuploidy. Early or late measurements can lead to incorrect assessments.
- **Option B:** This option is not specified, but if it implies a range that does not align with 11-14 weeks, it would be incorrect for the reasons mentioned above.
- **Option D:** Similarly, if this option suggests a gestational age outside the 11-14 week window, it would not be appropriate for nuchal translucency measurement.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the nuchal translucency measurement should be performed by a trained professional using a strict protocol to ensure accuracy. An increased nuchal translucency thickness is associated with a higher risk of not only Down syndrome but also other aneuploidies, cardiac malformations, and genetic syndromes.

## **Correct Answer:** B. 11–14 weeks.