Nuchal Translucency Scan: Purpose and Timing
The nuchal translucency (NT) scan is a critical ultrasound screening performed between 11 and 14 weeks of gestation to assess the risk of fetal aneuploidy, genetic syndromes, structural anomalies, and potential intrauterine demise. 1
Definition and Purpose
- Nuchal translucency is defined as the hypo-echoic space (fluid collection) between the overlying skin and underlying soft tissues of the posterior cervical spine of the first trimester fetus 1
- The primary purpose of NT measurement is to screen for chromosomal abnormalities, particularly Down syndrome (trisomy 21) and other aneuploidies 1, 2
- When combined with maternal age and serum markers (free beta-hCG and PAPP-A), NT screening can detect approximately 90% of chromosomal defects with a 5% false-positive rate 2
- NT screening also helps identify fetuses at risk for major cardiac defects, skeletal dysplasias, and genetic syndromes even with normal chromosomes 2
Optimal Timing
- NT scanning must be performed specifically between 11+0 and 14+0 weeks of gestation 1, 3
- The measurement is crown-rump length dependent, making the gestational age window critical for accurate assessment 1
- Optimal visualization and measurement accuracy improves after 10 weeks gestation 4
- The scan should not be performed before 11 weeks or after 14 weeks as the NT measurement loses its predictive value outside this window 1
Measurement Technique and Interpretation
- NT increases with crown-rump length, so gestational age must be considered when determining if a measurement is abnormal 1
- An increased NT is defined as ≥3 mm or above the 99th percentile for the crown-rump length 1
- Standardized measurement techniques are essential for accuracy and have been established by organizations like the Fetal Medicine Foundation and Nuchal Translucency Quality Review 1
- Quality assurance programs are important due to inter- and intra-observer variability in measurements 1
Clinical Significance and Management
- A measurement of ≥3 mm places the fetus at sufficient risk that genetic counseling and invasive diagnostic testing should be promptly offered 1
- About one-third of fetuses with NT ≥3 mm will have chromosomal abnormalities, with half of these being trisomy 21 1
- In chromosomally normal fetuses with increased NT, there remains an elevated risk of structural anomalies, particularly cardiac defects 1
- For twin pregnancies, NT is particularly valuable as it allows individual assessment of each fetus 1
- In dichorionic twins, each fetus has an independent risk reflected by its own NT measurement 1
- In monochorionic twins, a pregnancy-specific risk is calculated using the average NT of both fetuses 1
Follow-up Recommendations
- Patients with increased NT (≥3 mm) should be offered:
- Genetic counseling 1
- Invasive diagnostic testing (chorionic villus sampling or amniocentesis) 1
- Cell-free fetal DNA screening as an alternative, though this only detects trisomies 21,18,13 and sex chromosome abnormalities 1
- Detailed fetal anatomic survey in the second trimester 1
- Fetal echocardiography at 18-22 weeks 1
Limitations and Considerations
- NT measurement alone detects only about 70% of trisomy 21 fetuses and should be combined with other markers 1
- First trimester ultrasound for NT should not be performed solely for nuchal translucency screening in patients with negative cell-free fetal DNA results 1
- The sensitivity of ultrasound at 11-14 weeks for detecting congenital anomalies is only about 50% compared to 75% at 18-22 weeks 1
- Technical challenges may limit successful measurement in some cases, with success rates improving after 10 weeks gestation 4
NT screening represents an important early opportunity to identify pregnancies at increased risk for adverse outcomes, allowing for timely counseling and appropriate follow-up testing to optimize pregnancy management and outcomes.