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Nuchal Translucency (NT) Scan in Antenatal Ultrasound

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1. Definition

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2. Physiologic Basis / Pathogenesis

• Abnormal or delayed lymphangiogenesis (primary lymphatic defect)
• Cardiac anomalies with abnormal ductus venosus flow
• Extracellular matrix abnormalities
• Fetal infection
• Chromosomal aneuploidy

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3. Timing and Gestational Window

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4. Technique of NT Measurement (Standardized Protocol)

• Obtain a midsagittal view of the head, neck, and upper thorax
• The view should reveal: tip of the nose, third and fourth ventricles of the brain, and the palate
• The nuchal skin must be seen posteriorly
• The orbit, zygoma, and heart should NOT be visible
• The image must be magnified so that the fetal head, neck, and upper thorax fill most of the image

• The fetal head must be in line with the spine
• The neck must be in a neutral position - not flexed or extended
• Amniotic fluid must be visible between the chin and the anterior chest wall

• The entire NT line must be visible and crisp (not thick or fuzzy)
• Gain is typically turned down to make the nuchal line appear sharp
• The amnion must be seen as a separate distinct line from the NT (both appear as thin membranes at this age - wait for fetal movement or ask the mother to cough to separate them)

• Use crossbar calipers (+)
• Placed at the widest part of the NT
• Placed perpendicular to the long axis of the fetus
• Placed on the inner border of the nuchal line, immediately adjacent to the nuchal space
• The crossbar of the calipers must not protrude into the nuchal space

• Take multiple measurements; report the largest of several technically good measurements
• Report in millimeters to one decimal place (e.g., 1.2 mm, not 1.21 mm)
• Semiautomated measurement is acceptable if image quality requirements are met and the practitioner verifies caliper placement

• If a nuchal cord is present, measure the NT above and below the cord and average the two values
• This is the ONLY situation where averaging is used

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5. Crown-Rump Length (CRL) Measurement

• Measured on a midsagittal view of the fetus in a neutral position
• Calipers placed on the skin edge of the fetal head and rump (outer edges)
• The average of three good measurements is used in conjunction with the NT and serum analytes
• CRL determines which gestational age-specific NT normative value to compare against

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6. NT Values and Interpretation

• Normal: below 95th percentile for CRL
• Borderline/elevated: at or above the 95th percentile (approx. 2.5-3.0 mm at 12 weeks, varies by CRL)
• Significantly enlarged: ≥3.5 mm; risk of major cardiac defects rises steeply

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7. First-Trimester Combined Screening (The Full NT Scan Protocol)

1. NT measurement (ultrasound)
2. Serum PAPP-A (pregnancy-associated plasma protein-A) - reduced in Down syndrome (mean 0.4 MoM)
3. Free β-hCG - elevated in Down syndrome (mean 1.8 MoM)
4. Maternal age

• NT alone: ~70-75% for trisomy 21 at 5% FPR
• Combined first-trimester screen (NT + PAPP-A + free β-hCG): ~85-90% for trisomy 21 at 5% FPR (some studies report >90%)
• NT + serum analytes together: detect over 90% of major autosomal trisomies at a false-positive rate of 4%

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8. Additional First-Trimester Ultrasound Markers

a. Nasal Bone (NB)

• Absence of ossification of the fetal nasal bone is a marker for aneuploidy
• Absent NB in:
  • 69% of trisomy 21 fetuses
  • 55% of trisomy 18 fetuses
  • 34% of trisomy 13 fetuses
  • 11% of monosomy X fetuses
  • Only 0.6-2.6% of euploid fetuses
• Varies by race: absent NB in 2.2% Caucasians, 9.0% Afro-Caribbeans (euploid)
• Absence increases with larger NT measurements

b. Ductus Venosus (DV) Waveform

• Absent or reversed "a" wave in the DV suggests aneuploidy or cardiac dysfunction

c. Tricuspid Regurgitation

• Abnormal tricuspid flow is associated with cardiac defects and trisomy 21

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9. Associated Chromosomal Anomalies

• Trisomy 21 (Down syndrome) - most common; increased NT is the hallmark
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)
• 45,X (Turner syndrome / monosomy X) - often marked NT/cystic hygroma
• Triploidy

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10. Enlarged NT with Normal Karyotype

• Cardiac defects (most common - seen in 37% of cardiac anomaly fetuses vs. 6% with normal hearts)
• Diaphragmatic hernia
• Omphalocele (abdominal wall defects)
• Skeletal anomalies
• Renal anomalies

• Chromosomal microarray (CMA) shows an additional 4% yield for isolated enlarged NT; 7% when other anomalies are detected
• Most common CMA finding: 22q11.2 deletion
• Copy number variants (CNVs)
• RASopathy disorders - particularly Noonan syndrome (should be specifically tested when CMA is negative)
• Single-gene disorders

• In approximately 90% of cases where NT >99th percentile but <4.5 mm with normal karyotype and normal detailed sonogram, the infant is healthy with no developmental delay
• The larger the NT, the higher the residual risk of structural or genetic disorder

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11. Cardiac Follow-Up After Enlarged NT

• NT >99th percentile warrants fetal echocardiogram (typically at 20-22 weeks, but increasingly performed at 16-20 weeks given improved resolution)
• NT ≥5.5 mm: cardiac defect prevalence ~195/1000 (nearly 20%)
• Some centers perform first-trimester cardiac evaluation (detailed 4-chamber and outflow view) at the time of NT scan, identifying major cardiac defects before 14 weeks

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12. Combined and Sequential Screening Strategies

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13. Quality Assurance in NT Scanning

• In the United States: Perinatal Quality Foundation NT Quality Review (NTQR) program (initiated 2005)
• In the UK: Fetal Medicine Foundation (FMF) credentialing
• These programs: teach standardized mechanics, perform image review, and monitor provider performance longitudinally
• Only credentialed providers should perform and report NT measurements

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14. Role of Cell-Free DNA (cfDNA) vs NT Scan

• cfDNA has very high sensitivity for trisomies 21, 18, 13 and sex chromosome aneuploidies
• However, combined first-trimester and sequential screening detect a larger proportion of ALL chromosomal aneuploidies than currently available cfDNA platforms when followed by diagnostic genetic testing (NT + serum catches broader spectrum)
• cfDNA may be falsely reassuring in the setting of structural defects - NT-based screening may detect issues cfDNA misses
• The NT scan also provides additional structural survey of the fetus that cfDNA cannot

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15. NT in Multiple Gestations

• NT measurement is still performed in twins but interpretation requires care
• Monochorionic twins: enlarged NT in one or both twins may indicate twin-twin transfusion syndrome (TTTS) developing rather than aneuploidy - this reduces specificity
• NT in monochorionic twins has limited utility as a TTTS screening test in isolation
• CRL discordance in both monochorionic and dichorionic twins is associated with adverse perinatal outcome

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16. Management Algorithm Based on NT Result

NT Result
    │
    ├── Normal (<95th %ile) + normal serum markers
    │       → Routine antenatal care; low risk reassurance
    │
    ├── Screen positive (risk ≥1:270 or lab threshold)
    │       → Counseling; offer diagnostic testing:
    │           - CVS (10-13 weeks) for karyotype/CMA
    │           - Amniocentesis (15-20 weeks)
    │           - cfDNA (if patient declines invasive)
    │
    ├── NT 95th %ile to 3.4 mm, normal chromosomes
    │       → Detailed anatomy scan at 18-20 weeks
    │       → Fetal echo at 20-22 weeks
    │
    └── NT ≥3.5 mm or cystic hygroma
            → Immediate genetic counseling
            → Offer CVS or amniocentesis + CMA
            → If CMA negative, consider exome sequencing (structural/hydrops)
            → Noonan/RASopathy panel
            → Fetal echocardiography
            → Serial growth scans

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Summary of Key Points

1. NT is the fluid behind the fetal neck measured between 11+0 and 13+6 weeks (CRL 45-84 mm)
2. It is measured in a strict midsagittal plane with the neck neutral, calipers on inner borders, perpendicular to the fetal axis
3. Combined with PAPP-A and free β-hCG (first-trimester combined screen): detects >90% of major trisomies at 4% FPR
4. NT increases with CRL - results must be interpreted with gestational age-specific percentile charts
5. Enlarged NT ≥95th percentile warrants: CMA, fetal echo, detailed anatomy scan
6. Most common structural association: cardiac defects
7. Most common CMA finding with isolated enlarged NT: 22q11.2 deletion
8. With normal karyotype and normal detailed scan, ~90% of fetuses with NT >99th %ile but <4.5 mm have normal outcomes
9. Additional markers (nasal bone, DV waveform, tricuspid regurgitation) increase detection rates when assessed by credentialed providers
10. Quality assurance and sonographer credentialing are mandatory for reliable NT programs

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Recent Evidence Note: A 2025 meta-analysis (PMID: 40519157, Ultrasound Obstet Gynecol) specifically examined outcomes of fetuses with early increased NT, confirming the prognostic significance and supporting the above management framework. No major contradictions to current textbook guidance were identified.

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Nuchal Translucency (NT) Scan - Complete Guide

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1. What is NT Scan?

• It is part of the first-trimester antenatal ultrasound (ANC USG)
• Performed between 11 weeks 0 days to 13 weeks 6 days of gestation
• CRL (Crown-Rump Length) must be between 45 mm and 84 mm
• Combined with maternal serum markers to give a patient-specific risk for chromosomal aneuploidy

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2. Purpose / Why It Is Done

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3. Physiology and Pathogenesis

• Abnormal or delayed lymphangiogenesis (primary lymphatic defect)
• Cardiac anomalies with abnormal ductus venosus blood flow
• Extracellular matrix abnormalities
• Fetal infection
• Chromosomal aneuploidy

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4. How the NT Measurement is Performed

Step-by-Step Technique (Standardized Protocol)

• Strict midsagittal plane of the fetal head, neck, and upper thorax
• Must see: tip of the nose, third and fourth ventricles, palate, posterior nuchal skin
• Must NOT see: orbit, zygoma, or heart

• Fetal head, neck, and upper thorax must fill most of the screen

• Head in line with spine
• Neck in neutral position (neither flexed nor hyperextended)
• Amniotic fluid visible between chin and chest

• Both appear as thin white lines at this age
• Wait for fetal movement, or ask the mother to cough, to separate the amnion from the fetal neck skin

• Use crossbar (+) calipers only
• Place at the widest part of the NT
• Place perpendicular to the long axis of the fetus
• Place on the inner border of the nuchal line
• Crossbar must not protrude into the fluid space

• Take multiple measurements
• Report the largest technically valid measurement
• Report in mm to one decimal place (e.g., 2.4 mm, not 2.41 mm)

• Measure NT above and below the cord separately
• Report the average of those two values
• This is the ONLY situation where averaging is permitted

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5. CRL Measurement (Always Done Alongside NT)

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6. Normal Values and Cutoffs

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7. First-Trimester Combined Screening

• ~85-90% detection of trisomy 21 at 5% false-positive rate
• >90% detection of major autosomal trisomies overall at 4% FPR

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8. Additional Ultrasound Markers Assessed at NT Scan

Nasal Bone (NB)

• Absent nasal bone = marker for aneuploidy
• Absent in: 69% trisomy 21, 55% trisomy 18, 34% trisomy 13, 11% monosomy X
• Absent in only 0.6-2.6% of chromosomally normal fetuses
• Varies by race: Afro-Caribbean fetuses have higher rate of absent NB even when normal

Ductus Venosus (DV) Waveform

• Absent or reversed "a" wave = associated with aneuploidy and cardiac dysfunction

Tricuspid Regurgitation

• Abnormal tricuspid flow at NT scan = marker for trisomy 21 and cardiac defects

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9. Chromosomal Conditions Detected

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10. Risk Table for Down Syndrome by Maternal Age (at 12 Weeks)

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11. What Enlarged NT Means When Chromosomes Are Normal

• Cardiac defects (most common - found in 37% of fetuses with cardiac anomalies)
• Diaphragmatic hernia
• Omphalocele
• Skeletal dysplasia
• Renal anomalies

• Chromosomal microarray (CMA) - adds 4% additional yield for isolated enlarged NT; 7% when other anomalies present
• Most common finding: 22q11.2 deletion
• Noonan syndrome / RASopathy - test specifically when CMA is negative
• Whole exome sequencing - consider if hydrops or multiple structural anomalies

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12. Prognosis with Isolated Enlarged NT + Normal Karyotype

• When NT is >99th percentile but < 4.5 mm, karyotype is normal, and detailed scan is normal:
• ~90% of these fetuses are born healthy with normal development
• Risk of adverse outcome increases steeply as NT exceeds 4.5-5.5 mm

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13. Screening Strategies That Include NT

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14. Management Algorithm

NT Scan Result
│
├── Normal NT + Normal serum → Routine ANC, reassure
│
├── Screen Positive (risk ≥1:250 or ≥1:300)
│     → Genetic counseling
│     → Offer: CVS (10-13 wks) OR Amniocentesis (15-20 wks)
│     → cfDNA if patient declines invasive testing
│
├── NT 95th %ile to 3.4 mm + Normal chromosomes
│     → Detailed anatomy scan at 18-20 weeks
│     → Fetal echocardiography at 20-22 weeks
│
└── NT ≥ 3.5 mm or Cystic Hygroma
      → Urgent genetic counseling
      → CVS / Amniocentesis + Chromosomal Microarray (CMA)
      → If CMA negative → Noonan/RASopathy panel
      → If hydrops/multiple anomalies → Exome sequencing
      → Fetal echocardiography
      → Serial growth and anatomy scans

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15. NT in Twin Pregnancies

• NT is still measured in both twins
• Monochorionic twins: enlarged NT in one or both may represent early TTTS rather than aneuploidy - interpret with caution
• CRL discordance between twins (both mono and dichorionic) is associated with adverse perinatal outcome
• NT screening still has value in twins but has limited use as a standalone TTTS screening test

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16. Quality Assurance - Why Credentialing Matters

• USA: Perinatal Quality Foundation NTQR (NT Quality Review) program since 2005
• UK: Fetal Medicine Foundation (FMF) credentialing
• These programs:
  • Train providers in standardized technique
  • Review submitted images for quality
  • Monitor each provider's measurements over time against expected values
  • Require re-certification if measurements drift

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17. NT vs Cell-Free DNA (cfDNA)

• cfDNA may be falsely reassuring when structural defects are present
• Combined NT + serum screening detects a broader range of chromosomal conditions than cfDNA platforms, especially when followed by diagnostic testing

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Summary of 10 Key Points

1. NT = fluid behind fetal neck, measured at 11+0 to 13+6 weeks, CRL 45-84 mm
2. Strict midsagittal view, neck neutral, calipers on inner borders perpendicular to fetal axis
3. Combined with PAPP-A + free β-hCG: detects >90% of major trisomies at 4% FPR
4. NT is CRL-dependent - always interpret with percentile charts, not a fixed cutoff
5. Enlarged NT ≥95th percentile: offer CMA, fetal echo, detailed anatomy scan
6. Most common structural association: cardiac defects
7. Most common CMA finding with enlarged NT: 22q11.2 deletion
8. Isolated enlarged NT + normal karyotype + normal scan: ~90% healthy outcome
9. Additional markers (nasal bone, DV flow, tricuspid regurg) increase sensitivity
10. Sonographer credentialing (NTQR/FMF) is mandatory for program reliability