The nuchal fold (NF) thickness is a measurement performed on prenatal ultrasound, and is the distance from the outer edge of the occipital bone to the outer edge of the skin in the midline. While both measurements are at the level of the fetal head or neck, a nuchal fold thickness, which is only performed in the second trimester, should not be confused with a first trimester nuchal translucency (NT) measurement.
If an enlarged second trimester nuchal fold measurement is obtained, next steps should include:
Second trimester thickened nuchal fold has a high specificity for aneuploidy. ACOG defines an abnormal nuchal fold as ≥ 6mm in the 2nd trimester (typically performed between 15w0d and 22w6d). The SOGC (Society of Obstetricians and Gynecologists of Canada) considers a nuchal fold to be abnormal when the measurement is ≥ 5 mm at 16-18 weeks, or ≥ to 6 mm at 18-24 weeks. It is the most powerful second trimester sonographic marker for Trisomy 21 / Down syndrome.
ACOG Practice Bulletin 226: Screening for Fetal Chromosomal Abnormalities
SOGC: Fetal Soft Markers in Obstetric Ultrasound
ACOG Practice Bulletin No. 162: Prenatal Diagnostic Testing for Genetic Disorders
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