Increased Nuchal Translucency & Genetic RISK
Thickness matters — exponentially
Risk does not rise linearly. Once NT crosses 3.5 mm, the slope of risk increase becomes steep.
Septation changes the biology
- Non-septated NT → often hemodynamic/transient.
- Septated NT → implies lymphatic developmental abnormality.
- Large + septated = cystic hygroma physiology, not just “big NT.”
Chromosomal distribution shifts with thickness
- 3.5–4.5 mm → more T21
- 5 mm → rising Turner, T18, T13
- Very large septated NT → Turner predominates
If karyotype/CMA is normal
Residual risk depends heavily on:
- NT thickness
- Septation
- Cardiac findings
- Evolution over time (transient vs persistent)
For example:
- 3.8 mm, non-septated, resolves by 14 weeks → >90% chance of normal outcome.
- 6 mm, septated, persists → high risk even if CMA normal.
NT Thickness, Septation & Genetic RISK
| NT Thickness (mm) | Septation | Aneuploidy Risk | Major Structural Defect Risk | Genetic Syndrome (RASopathy, single-gene) | IUFD Risk | Overall Prognostic Impression |
|---|---|---|---|---|---|---|
| 2.5–3.4 mm | No | ~1–3% | ~2–4% | <1% | <1% | Often physiologic variant |
| 2.5–3.4 mm | Yes | ~5–10% | ~5–8% | ~2–3% | 1–2% | Mildly concerning |
| 3.5–4.4 mm | No | ~10–20% | ~8–15% | ~3–5% | 2–5% | Moderate risk |
| 3.5–4.4 mm | Yes | ~25–40% | ~15–25% | ~5–10% | 5–10% | High risk |
| 4.5–5.4 mm | No | ~25–40% | ~20–30% | ~8–15% | 5–15% | High risk |
| 4.5–5.4 mm | Yes | ~50–70% | ~30–50% | ~15–20% | 15–25% | Very high risk |
| 5.5–6.4 mm | No | ~40–60% | ~30–50% | ~15–20% | 15–30% | Very high risk |
| 5.5–6.4 mm | Yes | ~70–85% | ~40–60% | ~20–30% | 25–40% | Poor prognosis |
| ≥6.5 mm | No | ~50–70% | ~40–60% | ~20–30% | 30–50% | Severe risk |
| ≥6.5 mm | Yes (cystic hygroma pattern) | 70–90% | 50–70% | 25–35% | 40–60% | Very poor prognosis |
Pathophysiology
| Pattern | Likely Mechanism |
|---|---|
| Mild non-septated | Transient hemodynamic imbalance |
| Moderate persistent | Cardiac preload/afterload imbalance |
| Large septated | Lymphatic developmental failure |
| Massive + hydrops | Cardiac failure or chromosomal |
Mild isolated NT (3.0–3.4 mm, non-septated)
With a normal CMA:
- Risk is only slightly above baseline
- Prognosis is excellent
- Most babies are normal
This group often behaves like a physiologic fluid imbalance.
NT 3.5–4.5 mm, no septation
After normal CMA:
- Residual risk is mainly cardiac
- RASopathy risk exists but is modest
- If NT resolves and fetal echo is normal → outcome approaches 90%
This is the “transitional zone” group.
Large NT (>4.5 mm)
- Lymphatic development becomes more relevant
- Cardiac disease risk rises
- Single-gene disorders (especially RASopathies) become meaningful
Even with normal CMA, this is not low risk.
Septated NT / Cystic Hygroma
If septations are present:
- Think primary lymphatic dysplasia physiology
- Risk remains significant even if chromosomes are normal
- Resolution improves outlook but does not normalize risk
If CMA is normal:
- Small NT increase → prognosis very reassuring.
- Moderate NT (3.5–4.5) → mostly good, but fetal echo is key.
- Large NT (>4.5) → meaningful residual risk remains.
- Septated NT → risk remains elevated even with normal genetics.
- Resolution improves outlook substantially.