Prenatal Predictors of Adverse Pregnancy Outcome in Fetuses with Isolated Increased NT and Normal Genetic Testing

Background

Overall Risk of Adverse Outcome After Normal Genetics

Key Prenatal Predictors of Adverse Outcome

1. NT Thickness (most powerful predictor)

• The single strongest prenatal predictor. Risk increases non-linearly with NT size.
• NT > 6 mm carries a dramatically higher adverse outcome rate even after normal genetics: residual morbidity-related risk of ~33-77% depending on study population.
• In the Danish nationwide cohort (Gadsbøll et al., 2025, PMID 40017000), unaffected live birth prevalence dropped from 87% (NT 3.5-4.4 mm, normal CMA) to 29% (NT ≥ 6.5 mm, normal CMA) - underscoring that normal CMA does not normalize prognosis in very high NT.

2. Structural Anomalies on Follow-up Ultrasound

• Identification of structural anomalies (cardiac defects, diaphragmatic hernia, hydrops, skeletal dysplasia) at the 16-week or mid-trimester scan is a major adverse predictor.
• Zhen et al. (2022, PMID 35188046) demonstrated that a 16-week detailed ultrasound after increased NT + normal CMA detected structural defects in 26.5% of cases - most of which would otherwise only be found at 20-24 weeks.
• A normal mid-trimester anomaly scan is highly reassuring, reducing residual risk substantially.

3. Fetal Cardiac Anomalies

• Congenital heart defects (CHDs) are among the most common structural abnormalities in fetuses with increased NT and normal chromosomes. Detailed fetal echocardiography at 18-22 weeks is recommended as a prenatal predictor tool.

4. RASopathy / Monogenic Syndrome Testing

• RASopathies (Noonan syndrome, LEOPARD syndrome, Costello syndrome, CFC syndrome) account for a significant proportion of adverse outcomes in fetuses with elevated NT and normal CMA.
• After both normal CMA AND normal RASopathy testing, Spataro et al. (2023) found residual morbidity-related risks dropped to 7.1%, 8.7%, and 33.3% for NT 3.5-4.5 mm, 4.5-6 mm, and > 6 mm groups, respectively.
• The persistence of elevated risk even after normal RASopathy testing at high NT values underscores unknown genetic etiologies (candidates: WES, WGS).

5. NT Trajectory (Normalization vs. Persistence)

• A critical predictor is whether NT normalizes or persists/worsens at follow-up.
• In the prospective study by the Dutch FMU group (Ultrasound Obstet Gynecol, DOI 10.1002/uog.27623):
  • Normalized NT: 14.3% adverse outcome
  • Persistently increased NT: 65.2% adverse outcome (OR 11.3, 95% CI 4.4-28.5, p < 0.001)
• Their previous retrospective cohort found 24% adverse (normalized) vs. 78% (persistently increased).

6. Additional First-Trimester Markers

• Absent/reversed nasal bone, abnormal ductus venosus flow, tricuspid regurgitation, and increased umbilical artery PI each independently increase adverse outcome risk even beyond NT alone.
• Jugular lymphatic sacs / cystic hygroma visible on first-trimester scan are associated with worse outcomes.

7. Maternal Age and Biochemistry

• Abnormal first-trimester biochemistry (low PAPP-A, elevated free beta-hCG) in combination with elevated NT has been associated with higher risk of adverse perinatal outcomes including growth restriction.

8. Fetal Hydrops

• Development of hydrops fetalis is a grave prognostic sign. It reflects lymphatic, cardiac, or oncotic failure and is associated with very high rates of perinatal loss.

The Role of Chromosomal Microarray vs. Conventional Karyotype

• CMA detects slightly more submicroscopic aberrations than conventional karyotyping.
• However, a normal CMA result does not substantially improve prognosis compared to a normal karyotype in pregnancies with increased NT.
• This emphasizes that post-CMA counseling must still convey a meaningful residual risk.

What a Normal Genetic Test Does NOT Exclude

• Monogenic disorders not covered by current panels (e.g., rare RASopathies, skeletal dysplasias)
• Structural anomalies (cardiac, renal, pulmonary)
• Spontaneous pregnancy loss (likely due to hemodynamic or lymphatic failure)
• Postnatal neurodevelopmental delay (even with live birth)
• Prenatal whole-exome sequencing (WES) is emerging as the next diagnostic step and may identify additional pathogenic variants in ~5-10% of cases with isolated increased NT and normal CMA

Clinical Management Implications

1. Serial NT monitoring to assess trajectory (normalization is reassuring)
2. 16-week detailed anatomy scan to identify early structural anomalies
3. Fetal echocardiography at 18-22 weeks
4. RASopathy gene panel (10-gene or expanded) after normal CMA
5. Consider WES where available, especially for NT > 6 mm with normal CMA + normal RASopathy panel
6. Multidisciplinary counseling using NT-specific residual risk tables

Key References

• Gadsbøll K et al. "Increased nuchal translucency thickness and normal chromosomal microarray: Danish nationwide cohort study." Ultrasound Obstet Gynecol. 2025. PMID 40017000
• Spataro E et al. "Increased nuchal translucency with normal karyotype and genomic microarray analysis: A multicenter observational study." Int J Gynaecol Obstet. 2023. PMID 36572018
• Zhen L et al. "The 16-week sonographic findings in fetuses with increased nuchal translucency and a normal array." J Matern Fetal Neonatal Med. 2022. PMID 35188046
• Bianchi B et al. "Pregnancy outcomes in patients with increased nuchal translucency using non-invasive prenatal testing and first trimester ultrasound." Eur J Obstet Gynecol. 2025. (Belgian retrospective cohort, NT ≥ 2.5 mm, Aug 2017-Dec 2023)
• Early increased NT prospective cohort - Ultrasound Obstet Gynecol, DOI 10.1002/uog.27623 (Dutch prospective study, NT normalization as predictor)
• Frontiers in Medicine 2021 - Isolated Increased NT, microarray yield and pregnancy outcome (28.8% adverse outcome in normal karyotype group)

Recent Clinical Guidelines: Prenatal Management of Increased NT with Normal Genetics

1. ISUOG Practice Guidelines (Updated 2023) - 11-14 Week Ultrasound Scan

• NT measurement is mandatory at the 11-14 week scan regardless of whether it is used for aneuploidy risk calculation, because increased NT flags structural and genetic anomalies beyond common trisomies
• NT is increased in up to 40% of fetuses with major cardiac abnormality - echocardiography is therefore mandatory in the follow-up pathway
• Cystic hygroma (septal nuchal edema / dilated jugular lymphatic sacs) carries a worse prognosis than smooth NT thickening and should be separately documented
• NT must be measured using standardized technique (midline sagittal plane, neutral neck position, caliper on inner-to-inner borders, CRL 45-84 mm)

2. ISUOG/Prenatal Diagnosis Consensus - Increased NT After Low-Risk NIPT (Kelley & Hui et al., 2021)

Genetic Testing Pathway

Ultrasound Follow-Up Protocol

• 16-week detailed scan: 85% favourable outcome if completely normal at 16 weeks; detects 31% structural anomaly rate when NT > 3.5 mm
• 20-week anomaly scan: Mandatory; 11% chance of atypical chromosomal abnormality if structural anomaly found vs. 3% if isolated
• Fetal echocardiography at 24 weeks: Recommended given high CHD association
• CVS preferred over amniocentesis when invasive testing is pursued (preserves more DNA for future testing)

Counseling Key Points (Guideline-Specified)

• Overall risk of adverse outcome for NT > 3.5 mm: ~1 in 3 (33%)
• Normal newborn examination at birth: no long-term follow-up routinely required
• Testing for RASopathies should be considered if NT was > 5.0 mm at 11-13 weeks and no alternative diagnosis by 16 weeks (15% yield in this subgroup)
• No international consensus on a single NT cutoff threshold after low-risk NIPT; both 3.5 mm (fixed) and 99th centile/1.9 MoM (gestational age-controlled) are used

3. NT Threshold Positions by Society

4. Highest-Tier Recent Evidence Informing Current Guidelines

WES Meta-Analysis - Di Girolamo et al., 2023 [PMID 37019452]

• 8.1% incremental yield of WES over normal karyotype + CMA in fetuses with increased NT overall
• In fetuses with isolated increased NT and normal anomaly scan: 3.87% pathogenic/likely pathogenic variants found by WES
• Variants of unknown significance (VUS): 4.27% additional
• WES yield was higher at NT > 5.5 mm: 55.3% among WES-positive cases had NT in this range
• Implication: WES should be offered, especially when NT > 5 mm and standard testing is normal

RASopathy Panel Meta-Analysis - Powel et al., 2025 [PMID 41000078]

• Isolated increased NT + normal karyotype/CMA: RASopathy panel yield = 3.3% (95% CI 0.6-7.4%)
• Non-isolated increased NT (with additional ultrasound anomalies): yield = 15.1%
• Cystic hygroma present: yield = 18.4%
• Cardiac abnormalities were the most common associated finding in positive cases
• Guideline implication: RASopathy testing is most impactful when NT is non-isolated or very high; yield is modest (~3%) for truly isolated NT

Early NT (<11 weeks) Meta-Analysis - Matarrelli, Khalil et al., 2025 [PMID 40519157]

• Composite adverse outcome: 42% for early increased NT (CRL < 45 mm)
• NT persisting at 11-14 week scan: adverse outcome in 64.2%
• NT resolving at 11-14 week scan: adverse outcome in 19.4%
• Structural anomaly on ultrasound: 5.9%; perinatal loss: 9.7%
• Implication: NT trajectory from early to 11-14 week scan is a clinically important predictor; persistence is a major adverse flag

5. Step-by-Step Guideline-Based Management Algorithm

NT ≥ 3.5 mm (or > 99th centile) at 11-14 weeks
         |
         ├─ NIPT / targeted cfDNA (if not already done)
         │
         ├─ Invasive testing: CVS → CMA (preferred over karyotype alone)
         │
         ├─ If CMA normal:
         │    ├─ 16-week detailed anatomy scan (detects ~31% structural anomalies)
         │    ├─ RASopathy gene panel (especially NT > 5 mm or associated anomalies)
         │    ├─ Consider WES (especially NT ≥ 5 mm, multiple anomalies, or no diagnosis)
         │    ├─ 20-week mid-trimester anatomy scan
         │    └─ Fetal echocardiography at 18-24 weeks
         │
         └─ If all above normal:
              ├─ Residual risk counseling (~7-25% by NT size)
              ├─ Serial growth scans
              └─ Normal newborn exam → no mandatory long-term follow-up

6. Emerging Guidance: Whole-Genome Sequencing (WGS)

Key Reference Summary