What are the next steps for a 32-year-old pregnant woman at 13 weeks and 1 day gestation with a nuchal translucency (NT) measurement of 3 millimeters?

From the Guidelines

The next step for a 32-year-old pregnant woman at 13 weeks and 1 day gestation with a nuchal translucency (NT) measurement of 3 millimeters is to undergo genetic counseling and invasive diagnostic testing, such as chorionic villus sampling (CVS), as the risk of chromosomal abnormalities is significantly increased. According to the American College of Radiology 1, a nuchal translucency measurement of ≥ 3 mm places the fetus at sufficient risk that genetic counseling and invasive diagnostic testing should be promptly offered.

The patient's NT measurement of 3mm indicates a higher risk for chromosomal abnormalities, particularly Down syndrome (trisomy 21), and other genetic conditions such as Noonan syndrome and 22q11.2 deletion syndrome 1. While cell-free DNA (cfDNA) screening can detect some of these abnormalities, it is essential to note that it only detects trisomy 21,18, and 13, and sex chromosome aneuploidies, and may miss other genetic causes of increased nuchal translucency 1.

Key considerations for the patient's management include:

• Genetic counseling to discuss the risks and benefits of invasive diagnostic testing
• Invasive diagnostic testing, such as CVS, to obtain a definitive diagnosis
• A detailed fetal anatomy ultrasound at 18-22 weeks to assess for structural abnormalities
• A fetal echocardiogram at 20-24 weeks to evaluate cardiac function
• Serial fetal surveillance with periodic ultrasound examinations to monitor fetal growth and well-being 1

It is crucial to emphasize that the increased NT does not necessarily mean the fetus has an abnormality, but further testing is essential to provide more definitive information for pregnancy management and preparation 1.

From the Research

Nuchal Translucency Measurement

• The nuchal translucency (NT) measurement of 3 millimeters is considered abnormal, as most authors have used a nuchal thickness of > or = 2.5 mm or > or = 3 mm to define abnormal 2.
• The presence of a thickened nuchal translucency is associated with chromosomal abnormality and perhaps with structural abnormality even when the karyotype is normal 2.

Next Steps

• Patients with increased nuchal translucency should be counseled by their obstetrician and prenatal diagnostic testing should be offered 2.
• A study suggests that the NT cut-off for invasive testing could be 3.0 mm (instead of 3.5 mm) because of the high risk of 1:7.4 for a chromosomal aberration 3.
• Non-invasive prenatal testing (NIPT) may be offered, but it is essential to note that the residual risk for missing a (sub)microscopic chromosome aberration depends on the NIPT approach 3.
• Invasive prenatal testing, such as chorionic villus sampling (CVS) or amniocentesis, may be recommended for patients with an NT measurement of 3 millimeters or more 2, 3.

Risk Assessment

• The risk of chromosomal aberrations in fetuses with an NT of 3.0-3.4 mm is approximately 1:7.4 (13.5%) 3.
• The residual risk for missing a (sub)microscopic chromosome aberration after a normal NIPT result is highest if NIPT was performed only for common trisomies (1:21) and may be substantially lowered if a genome-wide 10-Mb resolution NIPT test was offered (~1:464) 3.

Clinical Practice

• The measurement of fetal NT thickness at the 11-14-week scan has been combined with maternal age to provide an effective method of screening for trisomy 21 4.
• Increased NT can also identify a high proportion of other chromosomal abnormalities and is associated with major defects of the heart and great arteries, and a wide range of skeletal dysplasias and genetic syndromes 4.
• However, the effectiveness of NT measurement as a screening tool in everyday usage appears less impressive, and its uptake as a screening technique in a general ultrasound practice may be imprudent 5.