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).
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:
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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The nuchal translucency (NT)/Nasal bone (NB) scan is performed at 11-13 weeks of pregnancy. It measures clear fluid at the back of the baby's neck (nuchal translucency) and the nasal bone length. The purpose is to assess risk for chromosomal abnormalities, including trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome).
No, an NT scan is not 100% accurate. Combined screening (including mother's age, NT measurement, gestational age, blood tests and nasal bone) detects 87% of trisomy 21 cases at 11 weeks, 85% at 12 weeks and 82% at 13 weeks, with 5% false-positive rate. It's a screening test for risk assessment, not a definitive diagnosis.
The NT / NB scan's purpose is specifically to assess risk for chromosomal abnormalities like Down syndrome, Edwards syndrome, and Patau syndrome by measuring nuchal translucency and nasal bone length. Gender revelation is not a part of this screening test.
Increased NT is considered when it measures above the 95th percentile for the given crown rump length (CRL). The aneuploidy rate is directly proportional to the nuchal translucency value, but it doesn't provide specific normal measurement ranges. Risk calculation involves a combination of factors, including NT measurement, maternal age and blood test results.
