Which measurement is used to assess soft tissue thickness at the back of the fetal head and is typically limited to 20 weeks gestation?

• A. Nuchal thickness
• B. Biparietal diameter
• C. Head circumference
• D. Occipitofrontal diameter

Answer: (A)

Nuchal thickness (nuchal fold thickness) is the measurement of soft tissue behind the fetal neck. This is assessed during the late second trimester anatomy scan, typically around 20 weeks, because the posterior neck skin fold becomes a more reliable marker of abnormality at that stage. The technique involves a transverse view of the head and neck and placing calipers on the posterior neck skin from the occipital bone to the skin surface, in a neutral fetal position. A thicker-than-normal nuchal fold can raise concern for chromosomal abnormalities or other anomalies and may prompt further evaluation. This is distinct from nuchal translucency, which measures the fluid-filled space behind the neck in the first trimester.