Normal Nuchal Thickness Size 18–22 Weeks Ultrasound
The nuchal fold is a soft-tissue measurement taken at the posterior fetal neck during the second-trimester anomaly scan, reflecting the amount of subcutaneous fluid and tissue overlying the occiput. Accurate measurement between 18 and 22 weeks of gestation is a well-established screening marker for chromosomal abnormalities, most notably trisomy 21 (Down syndrome). A thickened nuchal fold is one of the most predictive sonographic soft markers for fetal aneuploidy in the mid-trimester.
Normal Reference Values
| Measurement |
|---|
| <6 mm |
Clinical Significance
A nuchal fold thickness of less than 6 mm is considered normal between 18 and 22 weeks of gestation. A measurement of 6 mm or greater is regarded as abnormal and is associated with a significantly increased risk of Down syndrome and other chromosomal aneuploidies. Studies have reported that a thickened nuchal fold carries one of the highest likelihood ratios among second-trimester soft markers for trisomy 21.
It is important to distinguish the nuchal fold from the first-trimester nuchal translucency, which is measured at 11–14 weeks. The nuchal fold is a discrete second-trimester measurement with its own normative range. A borderline or thickened measurement should prompt a detailed anatomical survey, genetic counseling, and consideration of invasive testing or cell-free fetal DNA analysis.
- Trisomy 21 (Down syndrome) — most common association
- Trisomy 18 (Edwards syndrome) — often accompanied by structural anomalies
- Turner syndrome (45,X) — may present with nuchal thickening or cystic hygroma
- Triploidy — consider alongside other anomalies
- Normal variant / technical artifact — incorrect plane or fetal position can falsely elevate measurement
Reference: Locatelli A, Piccoli MG, Vergani P et al. Critical appraisal of the use of nuchal fold thickness measurements for the prediction of Down syndrome. Am. J. Obstet. Gynecol. 2000;182 (1 Pt 1): 192-7.
Imaging Notes
On ultrasound, the nuchal fold is measured in a transverse axial plane at the level of the cavum septi pellucidi, thalami, and cerebellar hemispheres — the same plane used for head circumference — with the fetal occiput posteriorly positioned. Calipers are placed from the outer edge of the occipital bone to the outer edge of the skin surface. Accurate measurement requires the fetal spine to be posterior (6 o’clock position) to avoid obliquity artifact that can falsely increase the measurement. Gain settings should be optimized to clearly delineate the skin line from surrounding amniotic fluid, and measurements should be performed with the fetal head in a neutral, non-flexed position.