What is the nuchal translucency (NT) threshold for suspected Down syndrome?

From the Guidelines

A nuchal translucency (NT) measurement of ≥ 3 mm at 11-14 weeks of gestation is considered abnormal and warrants further evaluation, including genetic counseling and invasive diagnostic testing, due to the increased risk of chromosomal abnormalities, such as Down syndrome. During first trimester screening, the NT measurement evaluates the fluid collection at the back of the fetal neck, which tends to be increased in fetuses with chromosomal abnormalities 1. The specific risk assessment combines the NT measurement with other factors, but an NT measurement of ≥ 3 mm is a significant indicator of potential issues, with about one-third of fetuses with nuchal translucency thickness above this threshold having a chromosomal abnormality, and half of these being trisomy 21 1.

Some key points to consider in the management of pregnancies with increased nuchal translucency include:

• The importance of genetic counseling and the offer of invasive diagnostic testing, such as chorionic villus sampling or amniocentesis, for definitive diagnosis 1.
• The potential benefits and limitations of cell-free fetal DNA screening, which can detect trisomy 21,18, and 13, as well as sex chromosome aneuploidies, but may not identify other genetic causes of increased nuchal translucency 1.
• The recommendation for targeted genetic studies, including microarray analysis, in all cases of increased nuchal translucency, even in euploid fetuses, due to the potential for genetic variants consistent with conditions like Noonan syndrome 1.
• The value of an early fetal anatomic survey and fetal echocardiogram for the detection of major structural anomalies, which may be performed at the time an increased nuchal translucency is identified, and the importance of a second trimester anatomic survey and fetal echocardiogram for ongoing evaluation 1.

It is crucial to approach each case individually, considering the specific risk factors and the preferences of the patient, while prioritizing the detection and management of potential fetal anomalies and chromosomal abnormalities to optimize outcomes in terms of morbidity, mortality, and quality of life 1.

From the Research

Nuchal Translucency Limit for Suspected Downs Syndrome

• The nuchal translucency (NT) limit for suspected Downs syndrome varies depending on the study and the gestational age of the fetus 2.
• A study published in 2009 found that combined prenatal screening was always positive for Down syndrome when NT thickness exceeded 4.0 mm 2.
• However, the upper NT threshold value changes according to gestational age and maternal age 2.
• For example, in women aged 35 to 37 years, combined prenatal screening was always positive when NT exceeded 2.8 mm, 3.0 mm, and 3.4 mm at 11,12, and 13 weeks of gestation, respectively 2.
• A more recent study published in 2021 found that the improved precision of NT measurement requires a modification to the NT MoM truncation limits to maintain accurate risk estimation 3.
• The study suggested that the NT MoM upper truncation limit should be reduced from 2.5 to 2.0 MoM to correct for the overestimation of Down's syndrome risk 3.
• Another study published in 2015 found that screening based on maternal age, fetal NT, and additional ultrasound markers such as nasal bone, tricuspid flow, and ductus venosus flow resulted in a detection rate of about 80%, 87%, and 94% for a false positive rate of 3% 4.

Factors Affecting Nuchal Translucency Limit

• Gestational age: The upper NT threshold value changes according to gestational age 2.
• Maternal age: The upper NT threshold value changes according to maternal age 2.
• Precision of NT measurement: The improved precision of NT measurement requires a modification to the NT MoM truncation limits to maintain accurate risk estimation 3.
• Additional ultrasound markers: The use of additional ultrasound markers such as nasal bone, tricuspid flow, and ductus venosus flow can improve the detection rate of Down syndrome 4.

Implications for Screening

• The use of NT screening can increase the antenatal detection of Down syndrome compared to maternal age-based screening 5.
• The combination of NT screening with additional ultrasound markers can improve the detection rate of Down syndrome 4.
• The modification of NT MoM truncation limits can help maintain accurate risk estimation and reduce the number of invasive tests required 3.