The nuchal fold is a normal fold of skin at the back of the fetal neck seen during the second trimester of pregnancy.
It is measured only on a second-trimester ultrasound.
It (NF) is the distance from the outer edge of the occipital bone to the outer edge of the skin in the midline. It is measured at the level of the fetal head or neck.
It should not be confused with a first-trimester nuchal translucency (NT) measurement which is measured in the first trimester.
It can be differentiated from a cystic hygroma by the absence of fluid-filled loculations at the back of the neck.
Increased thickness indicates a soft marker for chromosomal aneuploidies and structural defects in the fetus.
What? if there is an increased nuchal fold thickness!
Increased nuchal fold thickness is a significant antenatal soft marker observed during the second-trimester ultrasound, typically between 15 and 22 weeks of gestation.
The nuchal fold is the thickness of the skin at the back of the fetal neck.
Its increased measurement (greater than 6 mm) can be associated with an increased risk of chromosomal abnormalities, such as Down syndrome (trisomy 21), as well as other genetic conditions and congenital heart defects.
Measurement of nuchal fold
The nuchal fold is measured in a transverse section of the fetal head at the level of the cerebellum and occipital bone.
A thickness between 4.0 and 5 mm at 22 weeks of gestation is considered normal.
It is considered abnormal if it measures 6 mm or more. After 22 weeks the measurement become unreliable.
Risk Assessment
While an increased thickness can indicate a higher risk, it is not diagnostic on its own.
It should be considered alongside other markers and risk factors, such as maternal age, serum screening results, and other ultrasound findings.
In a study,149 out of 10,416 singleton pregnancies had an increased nuchal fold thickness.
The study subjects included fetuses who were already at risk for aneuploidy due to maternal age or abnormal maternal serum alpha-fetoprotein (AFP) levels.
How to follow up?
If increased nuchal fold thickness is detected, further diagnostic testing, such as non-invasive prenatal testing (NIPT), amniocentesis, or detailed fetal echocardiography, may be recommended to assess the risk of chromosomal abnormalities and other structural anomalies.
A second-trimester thickened nuchal fold has a high specificity for aneuploidy.
ACOG defines an abnormal nuchal fold as ≥ 6mm between 15 and 20 weeks of gestation.
It is the most powerful second-trimester sonographic marker for Trisomy 21 i.e. Down’s syndrome.
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