The majority of fetuses with increased nuchal translucency (NT) detected at the routine first trimester scan at 11-13 weeks gestation are normal. Some of them may, however, be associated with chromosomal abnormalities, genetic syndromes, structural abnormalities
and fetal infections.
Among the chromosomal abnormalities, the commoner ones include trisomy 21 (Down syndrome), trisomy 18 (Edward syndrome), trisomy 13 (Patau syndrome) and monosomy X (Turner syndrome). The common ones are usually detected on non-invasive prenatal tests like Panorama test. Rarer ones include less common trisomies, monosomies, deletions, duplications, and copy number variants that can only be detected by invasive tests like chorionic villus sampling or amniocentesis with the placental or amniotic fluid samples sent for chromosomal microarray (CMA) test. Such chromosomal microarray tests can detect microdeletions or microduplications up to 100,000 bases.
Genetic syndromes involving smaller changes in the genetic code (< 100,000 bases) can only be detected by a more detailed test such as whole exome sequencing (WES). This is especially indicated if there are more abnormalities detected on ultrasound scan at 11-13 weeks or even later at 16-20 weeks.
Structural abnormalities such as heart abnormalities are also associated with fetuses with increased nuchal translucency. A scan by a paediatric cardiologist with experience in fetal scanning may help to detect many of such abnormalities.
Fetal infections such as toxoplasmosis and cytomegalovirus (CMV) may result from symptomatic or asymptomatic maternal infections. The yield for checking of these infections in fetuses with increased NT is, however, very low.