From the Guidelines
The primary purpose of the Nuchal Translucency (NT) ultrasound is to assess the risk of chromosomal abnormalities, such as Down syndrome, Edwards syndrome, and Patau syndrome, by measuring the fluid-filled space at the back of the developing baby's neck between 11 and 14 weeks of gestation. This test is a powerful and independent ultrasound marker for fetal aneuploidy, with numerous studies confirming a strong association between increased nuchal translucency and abnormal fetal karyotype 1. The NT ultrasound is usually combined with blood tests measuring pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG) in what's called the "first trimester combined screening" 1.
Key Points
- The NT ultrasound measures the hypo-echoic space between the overlying skin and underlying soft tissues of the posterior cervical spine of the first trimester fetus 1.
- An increased nuchal translucency thickness is associated with a higher risk of fetal aneuploidy, genetic syndromes, structural anomalies, and intrauterine demise 1.
- The detection rate for Down syndrome using NT measurement is approximately 70% with a 5% false-positive rate 1.
- The NT ultrasound also allows healthcare providers to confirm pregnancy dating, check for multiple pregnancies, and sometimes detect major structural abnormalities.
Clinical Implications
The NT ultrasound is a non-invasive procedure that helps identify pregnancies that might benefit from further diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis 1. It's essential to understand that this is a screening test that calculates risk rather than providing a definitive diagnosis, and abnormal results would require follow-up testing for confirmation. As recommended by the American College of Medical Genetics, first trimester screening (NT, PAPP-A, and hCG) is an acceptable, cost-effective approach for Down syndrome risk screening for women who present early in pregnancy (before 14 weeks’ gestation) 1.
From the Research
Purpose of Nuchal Translucency (NT) Ultrasound
The purpose of the Nuchal Translucency (NT) ultrasound is to:
- Screen for Down syndrome and other chromosomal abnormalities between 11-14 weeks of gestation 2, 3, 4, 5, 6
- Measure the thickness of the subcutaneous fluid behind the fetal neck, which can indicate an increased risk of chromosomal abnormalities 3, 4, 5
- Identify major cardiac defects, skeletal dysplasias, and genetic syndromes 3, 4
- Detect fetal structural defects, including cardiac defects associated with increased NT 3
- Provide an effective method of screening for trisomy 21, with a detection rate of about 75% for an invasive testing rate of 5% 3, 4
Benefits of NT Ultrasound
The benefits of NT ultrasound include:
- Early detection of chromosomal abnormalities, allowing for invasive prenatal diagnosis and termination of affected pregnancies if desired 2, 3, 4
- High sensitivity and specificity for detecting Down syndrome and other chromosomal abnormalities 2, 3, 4
- Ability to screen for chromosomal abnormalities in multiple gestations 2
- Early diagnosis of major fetal anomalies, allowing for appropriate management and planning 3
Importance of NT Ultrasound
The importance of NT ultrasound is highlighted by the fact that:
- 70% of children with Down syndrome are born to women under the age of 36, who are not typically eligible for screening based on maternal age alone 6
- NT ultrasound can detect a high proportion of other chromosomal abnormalities and major defects of the heart and great arteries 3, 4
- The measurement of NT thickness can be combined with maternal age and other markers to provide an effective method of screening for chromosomal abnormalities 2, 3, 4