Can trisomy 18 be detected on an early morphology scan?

Detection of Trisomy 18 on Early Morphology Scan

Trisomy 18 can be detected on early morphology scans, with detection rates of approximately 75-92% when performed at or before 14 weeks gestation, and nearly 100% detection when performed at 18-21 weeks gestation. 1

First Trimester Detection Capabilities

Early detection of trisomy 18 relies on several ultrasound markers:

• Nuchal translucency (NT): The most sensitive marker in early pregnancy

  • Median NT in trisomy 18 fetuses is approximately 4.8mm compared to 1.8mm in euploid fetuses 2
  • When NT measures ≥3mm, about one-third of fetuses will have a chromosomal abnormality 3

• Additional ultrasound markers that improve detection:

  • Abnormal nasal bone (present in 46.2% of aneuploid fetuses) 2
  • Abnormal tricuspid valve flow (34.1% of aneuploid fetuses) 2
  • Abnormal ductus venosus flow (68.1% of aneuploid fetuses) 2
  • Structural anomalies (present in 83.5% of aneuploid fetuses) 2

Detection Rates and Accuracy

• First trimester detection rates for trisomy 18:

  • 75.8% when using maternal age and NT alone 2
  • 90.1% when adding nasal bone, tricuspid valve flow, and ductus venosus flow assessment 2
  • 94.5% when including detailed early anomaly scan 2
  • 95.6% when combining all markers 2

• By gestational age:

  • ≤14 weeks: 92.7% detection rate 1
  • 18-21 weeks: 100% detection rate 1

Common Ultrasound Findings in Trisomy 18

Multiple structural abnormalities are typically present in trisomy 18 fetuses:

• First trimester (most sensitive finding: increased nuchal translucency - 75%) 4
• Second trimester (most sensitive finding: cardiac defects - 83%) 4
• Other common findings include:
  • Clenched hands with overlapping fingers
  • Growth restriction
  • Choroid plexus cysts (present in 30-50% of trisomy 18 fetuses) 3
  • Congenital heart defects 5

Limitations and Caveats

• Early ultrasound detection has limitations:

  • Sensitivity for detecting congenital anomalies at 11-14 weeks is only about 50% compared to 75% at 18-22 weeks 3
  • Some structural abnormalities may not be apparent until later in pregnancy
  • False positives and missed diagnoses can occur, necessitating caution 3

• Important note: When an increased NT or other markers are found, diagnostic testing should be offered rather than relying solely on screening:

  • Chorionic villus sampling (10-13 weeks)
  • Amniocentesis (≥15 weeks) 3

Screening vs. Diagnostic Testing

• Non-invasive prenatal screening (NIPS) is highly effective for trisomy 18:

  • Detection rate of 98.83% (95% CI = 95.45%-99.71%)
  • False-positive rate of 0.07% (95% CI = 0.03%-0.17%) 3
  • PPV of 65.8% (95% CI = 45.3%-81.7%) 3

• However, NIPS is a screening test and cannot replace diagnostic testing when structural abnormalities are identified on ultrasound.

• The ACMG strongly recommends NIPS over traditional screening methods for fetal trisomies, including trisomy 18 3

In conclusion, early morphology scans can effectively detect trisomy 18, particularly when multiple markers are assessed, but diagnostic testing remains necessary for confirmation when abnormalities are identified.