THICK NUCHAL FOLD

• Nuchal fold measurement is obtained from the outer edge of the occipital bone to the skin surface in the transaxial plane of the fetal head at the level of the cavum septi pellucidum and cerebellar hemisphere.
• Nuchal fold can be spuriously thickened by angling caudally (intersecting the inferior level of the cerebellum and occiput).
• This nuchal skin fold increases with advancing gestational age and ranges between 1 and 5 mm in normal fetuses between 14 and 21 weeks gestation.
• A nuchal skin fold thickness of ³6 mm is considered abnormal between 14 and 21 weeks.
• Excess skin along the back of the neck is well known in babies with Down syndrome (80% of neonates).
• There are many studies that have reported the relationship of a thickened nuchal fold with abnormal karyotypes.
• Benacerraf and co-workers 1985 (1,2)(nuchal fold ³6mm).
• Combined Studies 1987(3)
• 42% fetuses with Downs syndrome had thickened nuchal fold.
  False positive rate = 0.1%
  Sensitivity = 42% specificity 99.9%
• Kirk an co-workers 1992(4)
• Thickened nuchal fold associated with 40% of abnormal karyotypes and 0.4% of normal second trimester fetuses.
• Crane and Gray 1991(5)
• Identified 75% of fetuses with Downs Syndrome, using the thickened nuchal fold, with a false positive rate of 1.4% and a positive predictive value of 1/13 for the detection of affected fetuses.
• Recommend using 5mm thickening between 14-18 weeks and 6mm from 19-24 weeks. Sensitivity = 50%, false positive rate of £3%.
• Interobserver variability
• 1mm of each other with a 95% probability.
  Reproducibility was within 0.5mm 95% of the time in the first trimester(6).

REFERENCES

1. Benacerraf BR, Barss VA, Laboda LA. A sonographic sign for the detection in the second trimester of the fetus with Down's Syndrome Am J Obstet Gynecol 1985; 151:1078-1079
2. Benacerraf BR, Frigoletto FD, Laboda LA. Sonographic diagnosis of Down syndrome in the second trimester Am J Obstet Gynecol 1985; 153: 49-52
3. Benacerraf BR, Frigoletto FD, Cramer DW. Down syndrome: sonographic sign for diagnosis in the second-trimester fetus Radiology 1987; 163:811-813
4. Kirk JS, Comstock CH, Fassnacht MA et.al. Routine measurement of nuchal thickness in the second trimester. J Matern Fetal Med 1992; 1:82-86
5. Crane JP, Gray DL. Sonographically measured nuchal skinfold thickness as a screening tool for Down syndrome: results of a prospective clinical trial. Obstet Gynecol 1991; 77:533-536
6. Benacerraf BR. The second-trimester fetus with Down Syndrome: detection using sonographic features Ultrasound Obstet Gynecol 1996; 7:147-155