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, and structural abnormalities.
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). These common ones can usually be detected on non-invasive prenatal tests like Panorama test. Rarer ones include less common trisomies / monosomies, deletions and 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 respectively 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) or whole genome sequencing (WGS). Some of the genetic abnormalities include RASopathies (see https://rasopathiesnet.org/rasopathies/syndromes/) like Noonan syndrome, and skeletal dysplasia and others. There are some laboratories that allow panel testing for RASopathies, and panel testing for skeletal dysplasia. Alternatively, whole exome sequencing or whole genome sequencing may be performed for a more thorough search for genetic syndromes that may be associated with increased NT. This is especially indicated if the NT is at 4.5 mm or more, and 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 may also be associated with fetuses with increased nuchal translucency. A scan by a paediatric cardiologist with experience in fetal scanning may help to detect many, but not all, of such abnormalities.