From the Guidelines
A normal nuchal translucency (NT) measurement during pregnancy is typically considered to be below 3 mm between 11 and 14 weeks of gestation, as measurements at or above this threshold are associated with an increased risk of fetal aneuploidy, genetic syndromes, structural anomalies, and intrauterine demise 1.
Key Considerations for Nuchal Translucency Measurement
- The measurement should be performed by a certified sonographer or physician using proper technique, with the fetus in a neutral position and at a crown-rump length between 45-84 mm.
- Nuchal translucency is just one component of prenatal screening and is most informative when combined with maternal blood tests (PAPP-A and free beta-hCG) and maternal age to calculate an overall risk assessment.
- Even with a normal NT measurement, additional screening or diagnostic tests like NIPT (non-invasive prenatal testing) or amniocentesis might be recommended based on other risk factors.
- The NT measurement increases normally with gestational age, so results are always interpreted in relation to how far along the pregnancy is.
Implications of Abnormal Nuchal Translucency Measurements
- A nuchal translucency measurement of ≥ 3 mm places the fetus at sufficient risk that genetic counseling and invasive diagnostic testing should be promptly offered 1.
- In cases of increased nuchal translucency, the probability of fetal aneuploidy is higher when significant tricuspid regurgitation or reversed a-waves in the ductus venosus are present 1.
- The addition of ductus venosus Doppler assessment to nuchal translucency measurement improves the detection rate of trisomy 21 1.
Recommendations for Clinical Practice
- Nuchal translucency measurements should be interpreted in the context of gestational age and other prenatal screening results to provide an overall risk assessment for fetal aneuploidy and other adverse outcomes 1.
- In experienced centers, an early fetal anatomic survey may also be performed at the time an increased nuchal translucency is identified to detect major structural anomalies 1.
- Serial fetal surveillance is usually added to routine prenatal care with periodic US examinations to monitor fetal growth and well-being in cases of increased nuchal translucency 1.
From the Research
Normal Nuchal Translucency
- The size of nuchal translucency varies slightly with gestational age and crown-rump length, and is independent of maternal age 2.
- Most authors have used a nuchal thickness of > or = 2.5 mm or > or = 3 mm to define abnormal nuchal translucency 2.
- A normal nuchal translucency is typically considered to be less than 3 mm 2, 3.
- The presence of a thickened nuchal translucency is associated with chromosomal abnormality and perhaps with structural abnormality even when the karyotype is normal 2.
- Normal values of PAPP-A, free beta-hCG, and nuchal translucency are associated with a very low risk of pregnancy loss at less than 20 weeks 4.
Measurement and Screening
- Fetal nuchal translucency can be measured in most pregnant women in the first and early second trimester 2.
- The accuracy of nuchal translucency measurement varies between examiners and between patients, likely in relation to examiner skill and image resolution 2.
- Combining first-trimester ultrasonography with early serum screening is currently being investigated and may ultimately prove to be the most efficient means of screening for chromosomal anomaly 2.
- Simultaneous biochemical and ultrasound testing is an indispensable condition for prenatal diagnosis of trisomy 21 in the 1st trimester of pregnancy 5.
Association with Chromosomal Abnormalities
- A low serum PAPP-A or an increased nuchal translucency was seen in two-thirds of all pregnancies with chromosomal abnormalities 3.
- A nuchal translucency > or = 3 mm and depressed serum PAPP-A levels have a good predictive value in the detection of fetal aneuploidy at 10 to 13 weeks of pregnancy 3.
- The reported sensitivity for identification of trisomy 21 using nuchal translucency measurement has ranged from about 40% to 80% 2.