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Nuchal Thickness

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Nuchal thickness refers to increased soft tissue thickening on the posterior aspect of the neck measured in the early 2nd trimester between 16 and 20 weeks gestation. Beyond this gestation the measurement is unreliable as a marker for chromosomal abnormalities. It is also referred to as nuchal edema or nuchal skinfold thickness. It is a different measurement to that of nuchal translucency in the first trimester and it would appear that very few cases of increased nuchal translucency progress to nuchal thickening in the 2nd trimester. A measurement of 6mm or more is generally taken to indicate increased risk of aneuploidy. An association has been found with chromosomal defects, mainly trisomy 21 but also other trisomies, deletions, triploidy and Turner’s Syndrome. Early studies found it to be a poor and unreliable screening test for Down syndrome. However, recent work suggests it is useful as a ultrasonographic marker for aneuploidy, present in about 40% of cases of Trisomy 21. It may be the most sensitive and specific of all the second trimester ultrasonographic markers for Down Syndrome. Some groups have proposed the use of this marker in combination with others, such as femur length, to develop risk scoring systems. Each additional marker seen on ultrasound raises the risk of Down still further. Recently studies combining increased nuchal thickness, maternal age and urinary beta-HCG core fragment have been used to screen for Down with 80% sensitivity. Spontaneous resolution of nuchal thickening may occur after 20 weeks even in a chromosomally abnormal fetus, so resolution cannot be interpreted as reassuring.The scanning plane used in the assessment of nuchal thickness is crucial. In the 2nd trimester a modified axial or transverse plane of the fetal head (suboccipitobregmatic) is obtained, with the transducer angled posteriorly in order to simultaneously visualize the cavum septum pellucidum, cerebral peduncles, cerebellar hemispheres and the cisterna magna. In this plane measurement of nuchal thickness is obtained from the outer limit of the occipital bone to the outer aspect of the skin edge. A search should be made for additional ultrasonographic markers of fetal aneuploidy, such as reduced femur and humeral length, pyelectasis, hypoplasia of the fifth digit middle phalanx, small ear, echogenic bowel and echogenic ventricular foci as well as anomalies such as cardiac defects, omphalocoele, hydrops, and duodenal atresia. Title: Ultrasonographically adjusted midtrimester risk of trisomy 21 and significant chromosomal defects in advanced maternal age. Author: Bahado-Singh RO, et al. Journal: Am J Obstet Gynecol. Dec;175(6):1563-8. Year: 1996 Title: Early midtrimester fetal nuchal thickness: effectiveness as a marker of Down syndrome. Author: Borrell A, et al. Journal: Am J Obstet Gynecol. Jul;175(1):45-9. Year: 1996 Title: First and second trimester sonography: an American perspective. Author: Scorza WE, et al. Journal: Int J Fertil Menopausal Stud. May-Jun;41(3):288-92. Year: 1996 Title: Normal nuchal thickness in the midtrimester indicates reduced risk of Down syndrome in pregnancies with abnormal triple-screen results. Author: Bahado-Singh RO, et al. Journal: Am J Obstet Gynecol. Oct;173(4):1106-10. Year: 1995 Title: The resolving nuchal fold in second trimester fetuses: not necessarily reassuring. Author: Bromley B, et al. Journal: J Ultrasound Med. Mar;14(3):253-5. Year: 1995 Title: Sonographic measurement of nuchal skinfold thickness for detection of Down syndrome in the second-trimester fetus: a multicenter prospective study. The AFDPHE Study Group. Ass Author: Grandjean H, et al. Journal: Obstet Gynecol. Jan;85(1):103-6. Year: 1995 Title: Optimal nuchal skin-fold thresholds based on gestational age for prenatal detection of Down syndrome. Author: Gray DL, et al. Journal: Am J Obstet Gynecol. Nov;171(5):1282-6. Year: 1994 Title: Prospective multicenter study of second-trimester nuchal skinfold thickness in unaffected and Down syndrome pregnancies. Author: Donnenfeld AE, et al. Journal: Obstet Gynecol. Nov;84(5):844-7. Year: 1994

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