This is one of the most common reasons families are referred to a genetics clinic after first trimester screening. The American College of Obstetricians and Gynaecologists considers a nuchal translucency of 3.0 millimetres or more, or above the 99th percentile for the baby’s size, as increased. At this level, genetic counseling and diagnostic testing should be offered, even if the blood test and overall risk number look reassuring.

A thick nuchal translucency is not only linked to the common trisomies. It can also be an early sign of:

• Smaller chromosome changes for example, 22q11.2 deletion syndrome, which can affect the heart, immune system, and learning. These are found by chromosomal microarray.
• Single-gene conditions the best known are the RASopathies, a group that includes Noonan syndrome. These are caused by changes in genes such as PTPN11, SOS1, RAF1, and RIT1, and can affect the heart, growth, and development.
• Heart and skeletal differences Some babies with thick nuchal translucency have a heart condition or a skeletal dysplasia, a condition affecting bone growth.

When the chromosomes and microarray are normal, some centres offer a RASopathy gene panel or exome sequencing, which reads most of the baby’s genes at once. A 2023 review combining eight studies found that about 8 in every 100 babies with an isolated thick nuchal translucency and normal chromosome tests had a genetic diagnosis on exome sequencing. When the later anatomy scan was completely normal, this fell to about 4 in every 100. The chance of finding something rises as the measurement gets thicker.

There is also good news. Research shows that if the chromosome tests are normal and a detailed anatomy scan at 18 to 22 weeks shows no problems, the chance of a healthy baby is close to that of any other pregnancy. A detailed heart scan, called a fetal echocardiogram, is also usually advised. Most babies in this situation are born healthy.