NT/ NB SCAN: AN INSIGHT

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).

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

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

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• 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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