Answer:

Nuchal translucency (NT) refers to the temporary fluid accumulation at the back of a fetus's neck as its lymphatic system develops. An NT ultrasound is typically performed in the first trimester (11 to 13 weeks and 6 days) to ensure measurement accuracy. Before 11 weeks, the fetus is too small, and its body structures are not yet clear enough for precise measurement. From week 14, the fetal lymphatic system develops significantly, absorbing this fluid layer, causing it to disappear or become very thin, thus losing its value in predicting chromosomal abnormalities.

A nuchal translucency measurement greater than 3.5 mm can suggest an increased risk of Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), Turner syndrome, heart defects, or other gene-related genetic abnormalities.

While a thicker nuchal translucency increases the risk of Down syndrome, it is only an indicator, not a diagnostic method. Some cases show a very low measurement (for example, 1.8 mm) yet the fetus still has the condition. Conversely, some babies with a thick nuchal translucency are born completely healthy. This occurs because Down syndrome often causes fluid accumulation due to lymphatic system or heart issues, but not every fetus with Down syndrome clearly exhibits this on ultrasound images. Relying solely on ultrasound results can miss cases of chromosomal abnormalities that do not manifest through nuchal translucency thickness.

Doctor Long performs a nuchal translucency ultrasound for a pregnant woman. Illustration: Tam Anh District 7 General Clinic

To screen for Down syndrome in the first trimester, pregnant women can undergo non-invasive prenatal testing (NIPT) or a combined test (maternal age, nuchal translucency, and blood tests like PAPP-A and free Beta HCG). NIPT can detect 99,7% of Down syndrome cases, while the combined test detects approximately 90-95%. If nuchal translucency is over 3.5 mm or screening results indicate a high risk, doctors will advise the pregnant woman to undergo amniocentesis or chorionic villus sampling (CVS), bypassing screening tests like NIPT or the combined test to diagnose any genetic abnormalities.

The goal of Down syndrome screening is to provide early information to pregnant women and their families about the risk of the fetus having Down syndrome. This information helps families make appropriate decisions based on their circumstances, beliefs, and financial situation. It also allows families to prepare emotionally, and to research medical and community support available for children with Down syndrome, helping them develop their full potential and improve their quality of life later on.

Doctor Nguyen Hoang Long

Fetal Medicine Unit

Tam Anh District 7 General Clinic