Nuchal Translucency (NT)
The examination is more correctly called »early fetal morphology« because, in addition to risk assessment of chromosomal anomalies, we also check for any irregularities in the early development (abdominal wall, brain, extremities, internal organs). We look for the presence of a nasal bone. From the size of the fetus, we can determine the estimated date of delivery with great precision.
The examination is performed with a transabdominal ultrasound. The transvaginal transducer is only used exceptionally (fetal position, adiposity of the pregnant woman). The appropriate time is when the fetus size is around 45 to 84 mm (from 11 3/7 to 14 weeks of gestation).
Nuchal translucency is fluid at the back of the fetal neck in a subcutaneous space. It is increased in fetuses with chromosomal anomalies, congenital heart disease, and some genetic syndromes.
Nuchal translucency scan in a screening test. Establishing the presence or absence of the nasal bone detects around 80% of fetuses with chromosomal anomalies. Combined with the double marker test, it has up to 90% positive predictive value for fetal anomalies.
A result for each of the chromosomal aneuploidies (trisomy 21, 18, and 13) is better than
is interpreted as normal. The result is statistical and expressed as a risk. The result for each chromosomal anomaly depends on multiple factors, most notably the pregnant woman’s age, nuchal translucency, fetal heart rate, and fetal morphology.
When the risk is high, it has to be confirmed with diagnostic tests (chorionic villus biopsy or amniocentesis) and sometimes with cell-free DNA testing.