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NT/ NB SCAN: AN INSIGHT

January 9, 2024

The nuchal translucency (NT) / Nasal bone scan screens your baby for these abnormalities. This test is typically scheduled between weeks 11 and 13 of pregnancy.

NT/NB scan: Done in pregnancy at 11-13 weeks. This test measures the size of nuchal translucency, ie, clear fluid at the back of your baby’s neck along with the length of the nasal bone.

Purpose: To assess the risk for the chromosomal abnormalities which include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) or trisomy 13 (Patau syndrome).

Book an online appointment with Dr. Astha Sahni for Pregnancy & Antenatal related Scans.

An NT-NB scan / ultrasound scan at 11-13 weeks is done to assess following parameters:

  • To measure the crown rump length (must be 45 – 84 mm);
  • To measure the collection of fluid behind the fetal neck (nuchal translucency);
  • To measure intracranial translucency which depicts the fourth ventricle in mid-sagittal plane;
  • To examine the fetal nose and palate;
  • To measure the fetal heart rate;
  • To assess the flow of blood across the tricuspid valve of the fetal heart and the ductus venosus (DV).

Associated Blood test: Dual/ Double marker: A blood test advised along with the NT scan. The calculation of any chromosomal risk is based on combined screening including mother’s age, the NT measurement, the gestational age, blood tests ( double marker test ) and the nasal bone (presence/ absence).

Interpretation: Increased NT is considered when it measures >95th percentile for a given crown rump length (CRL). The rate of aneuploidy is directly proportional to the value of nuchal translucency:

  • <2 mm : risk of <1%.
  • 3.4 mm : risk of 7%
  • 3.5-4.4 mm : risk of 20%
  • 5.5-6.4 mm : risk of 50%
  • ≥8.5 mm : risk of 75%

To increase the clinical accuracy of nuchal lucency, it can be correlated with serum markers such as:

  • maternal B-HCG ·
  • alpha-fetoprotein (AFP)
  • pregnancy-associated plasma protein A (PAPP-A)
  • estriol/estriol

The combination of nuchal translucency thickness, PAPP-A, and hCG detects 87% of cases of trisomy 21 at 11 weeks, 85% at 12 weeks, and 82% at 13 weeks, with a 5% false-positive rate.

References : Fong K, Toi A, Salem S et al. Detection of Fetal Structural Abnormalities with US during Early Pregnancy. RadioGraphics. 2004;24(1):157-174.

Kenkhuis M, Bakker M, Bardi F et al. Effectiveness of 12-13-week scan for early diagnosis of fetal congenital anomalies in the cell-free DNA era. Ultrasound Obstet Gynecol. 2018;51(4):463-469.

Kurtz AB, Johnson PT. Obstetric and gynecologic ultrasound. Mosby. ISBN:0323008607.

Weissleder R, Wittenberg J, Harisinghani MMGH et-al. Primer of Diagnostic Imaging. Mosby. (2007) ISBN:0323040683.

Thilaganathan B, Sairam S, Michailidis G et-al. First trimester nuchal translucency: effective routine screening for Down's syndrome. Br J Radiol. 1999;72 (862): 946-8.

Van vugt JM, Van zalen-sprock RM, Kostense PJ. First-trimester nuchal translucency: a risk analysis on fetal chromosome abnormality. Radiology. 1996;200 (2): 537-40.

Pittrof R, Majid S. Chromosomal abnormalities and fetal nuchal translucency in first trimester. BMJ. 1992;304 (6838): 1384.

Josefsson A, Molander E, Selbing A. Nuchal translucency as a screening test for chromosomal abnormalities in a routine first trimester ultrasound examination. Acta Obstet Gynecol Scand. 1998;77 (5): 497-9.

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