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, but with significantly lower sensitivity compared to standard second-trimester ultrasound examinations. Early detection is possible through a combination of ultrasound markers and maternal serum screening, though detection rates are limited at earlier gestational ages.

Ultrasound Detection of Trisomy 18

First Trimester (11-14 weeks)

• Nuchal translucency (NT): When NT measures ≥3mm, approximately one-third of fetuses may have chromosomal abnormalities 1
• Structural anomalies: First-trimester detection includes:
  • Increased nuchal translucency (present in 91% of trisomy 18 cases) 2
  • Absent/hypoplastic nasal bone (53% of cases) 2
  • Omphalocele (21% of cases) 2
  • Abnormal hand posturing (6% of cases) 2
  • Early-onset growth restriction (26% of cases) 2
  • Cardiac abnormalities (4% of cases) 2

Detection Rate Limitations

• First-trimester ultrasound detection sensitivity for congenital anomalies is only about 50% compared to 75% at 18-22 weeks 1
• Some structural abnormalities associated with trisomy 18 may not be apparent until later in pregnancy 1

Second Trimester Detection (More Reliable)

• Choroid plexus cysts are present in 30-50% of trisomy 18 fetuses 3, 1
• Multiple structural anomalies are almost always evident in trisomy 18, including:
  • Structural heart defects
  • Clenched hands with overlapping fingers
  • Talipes (club foot)
  • Growth restriction
  • Polyhydramnios 3

Combined Screening Approaches

First Trimester Combined Screening

• Combining ultrasound findings with maternal serum markers (PAPP-A and free beta-hCG) between 11-14 weeks improves detection 3
• First trimester screening has a positive predictive value of approximately 27% for trisomy 18 4

Second Trimester Serum Screening

• Maternal serum screening using AFP and hCG has lower positive predictive value (approximately 2%) 4
• Trisomy 18 is associated with low levels of PAPP-A (median 0.08 MoM), AFP (median 0.61 MoM), and free beta-hCG (median 0.24 MoM) 5

Diagnostic Testing Recommendations

When abnormal findings are detected on early morphology scan:

• Non-Invasive Prenatal Screening (NIPS) should be offered as it has a high detection rate for trisomy 18 (98.83%) with a low false-positive rate (0.07%) 1

• Diagnostic testing should be considered when increased NT or other markers are found:

  • Chorionic villus sampling (10-13 weeks)
  • Amniocentesis (≥15 weeks) 1

Clinical Implications

• The typical pattern of trisomy 18 includes major and minor anomalies, prenatal growth deficiency, and increased risk of neonatal mortality 6
• Early detection allows for appropriate counseling and management planning
• When ultrasound findings suggest trisomy 18, diagnostic testing should be offered rather than relying solely on screening 1

Important Caveats

• False positives and missed diagnoses can occur, particularly in early scans 1
• Isolated soft markers (like choroid plexus cysts) have limited value without other findings 3
• Ultrasound scan for fetal anomalies remains the most effective screening test for trisomy 18 4