What is the normal thickness of the nuchal (back of the neck) fold?

From the Guidelines

Normal nuchal fold thickness varies depending on gestational age, with a measurement of less than 3mm considered normal in the first trimester (11-14 weeks) and less than 6mm in the second trimester (16-24 weeks) 1. The nuchal fold measurement is an important screening marker for chromosomal abnormalities, particularly Down syndrome (trisomy 21), and is taken via ultrasound at the back of the fetal neck, where fluid accumulation occurs. Some key points to consider include:

• The thickness of the nuchal fold naturally increases as pregnancy progresses, which is why age-specific normal ranges are used 1.
• An increased nuchal fold thickness can indicate a higher risk for chromosomal disorders, cardiac defects, or genetic syndromes, though it's not diagnostic on its own 1.
• If an abnormal measurement is found, additional testing such as NIPT (non-invasive prenatal testing), amniocentesis, or detailed anatomical ultrasound may be recommended to further evaluate the fetus 1.
• Regular prenatal care with appropriate screening is essential for monitoring fetal development and detecting potential complications early on 1. In terms of specific guidelines, the Society for Maternal-Fetal Medicine recommends counseling and discussion of options for noninvasive aneuploidy screening or diagnostic testing via amniocentesis for pregnant people with isolated thickened nuchal fold 1. Overall, the nuchal fold measurement is a valuable tool in prenatal screening, and its interpretation should be guided by the most recent and highest-quality evidence available 1.

From the Research

Nuchal Fold Thickness

The normal thickness of the nuchal fold can vary depending on several factors, including gestational age and fetal neck position.

• Studies have shown that nuchal fold thickness increases with gestational age 2, 3, 4, 5.
• At 15 to 21 weeks of gestation, the median nuchal fold thickness was 3.5 mm for fetuses with a flexed neck and 3.9 mm for those with an extended neck 2.
• A study found that the mean nuchal fold thickness in euploid fetuses was 3.2 +/- 0.9 mm, while in fetuses with trisomy 21 it was 4.7 +/- 1.6 mm 3.
• Another study established a regression equation for nuchal skin-fold thickness based on gestational age, cephalic index, presentation, and the presence or absence of nuchal cord 4.
• A retrospective cross-sectional study found that nuchal fold thickness increased with gestational age in a linear manner from 3.13 +/- 0.68 mm at 16 weeks to 5.08 +/- 0.76 mm at 24 weeks 5.

Factors Affecting Nuchal Fold Thickness

Several factors can affect nuchal fold thickness, including:

• Gestational age: nuchal fold thickness increases with gestational age 2, 3, 4, 5.
• Fetal neck position: nuchal fold thickness is affected by the position of the fetal neck, with flexed neck positions resulting in lower measurements 2.
• Cephalic index: there is a negative correlation between cephalic index and nuchal fold thickness 4.
• Presentation: breech presentation is associated with thicker nuchal skin-fold measurements than vertex presentation 4.
• Presence or absence of nuchal cord: the presence of a nuchal cord can increase nuchal skin-fold thickness 4.