Nuchal Thickening

Redundant soft tissue in the back of the neck area has been known to be a feature of newborns and fetuses with Down’s syndrome. A sonographic sign of a thickened nuchal fold is used as a marker of an increased risk of fetal Down’s syndrome in the second trimester, typically after 15-20 weeks of gestation. This measurement has remained the most sensitive and specific single marker for the midtrimester detection of Down’s syndrome.

Sonographic examination:

Fig 1, Fig 2, Fig 3

• The measurement is done using a transverse section of the fetal head, angled posteriorly to include the cerebellum and the occipital bone. The following landmarks must be identified: cavum septi pellucidi, cerebral peduncles, cerebellar hemispheres and cisterna magma. The measurement is made from the outside of the occipital bone to the outer skin edge.
• A soft tissue thickening of 6 mm or greater between 15 and 20 weeks of gestation is considered abnormal. However, the 95th percentile of nuchal fold thickness measured by transvaginal sonography at 14-16 weeks is 3.0 mm.
• The observer variability for this measurement is only 1 mm among experienced practitioners.
• Combining second trimester serum testing and nuchal fold thickness is substantially more effective than either serum screening or ultrasound alone.

Nuchal thickening:  Transcerebellar plane: thickened nuchal fold (arrowhead) (arrow = occipital bone)

Nuchal thickening:  Transcerebellar plane: thickened nuchal fold (arrow)

Nuchal thickness along with other sonomarkers during 15-20 weeks is incorporated into the sonographic scoring index for detection of Down’s syndrome in the second trimester as follows:

Sonographic index for Down’s syndrome in second trimester fetuses

Findings                                                 Score

Major anomaly                                          2

Nuchal fold >6 mm                                   2

Short femur                                               1

Short humerus                                           1

Pyelectasis                                                 1

Hyperechoic bowel                                   1

Echogenic intracardiac focus                    1

• This genetic sonogram scoring index was used to identify approximately 75% of fetuses with Down’s syndrome, with amniocentesis being recommended in 26.7% of a high-risk population.