What does it mean if the ductal arch is not visualized on the third anatomy scan attempt at 25 weeks pregnant?

Inability to Visualize the Ductal Arch at 25 Weeks May Indicate Conotruncal Cardiac Anomalies

The inability to visualize the ductal arch on a third anatomy scan attempt at 25 weeks pregnancy is concerning and suggests a possible conotruncal heart defect that requires prompt referral to fetal cardiology for comprehensive evaluation.

Clinical Significance of Non-Visualization

The ductal arch is an important cardiovascular structure that should be readily visible during second and third-trimester ultrasound examinations. Research shows that failure to visualize the ductal arch has significant diagnostic implications:

• The sagittal view of the ductal arch is considered a standard view in fetal echocardiography 1
• Non-visualization of the ductal arch is strongly associated with conotruncal anomalies with a likelihood ratio of 9.44 (95% CI: 5.8-15.5) 1
• In fetuses with normal cardiac anatomy, the ductal arch can be visualized in 93.1% of cases, compared to only 5.6% in those with conotruncal anomalies 1

Recommended Next Steps

1. Immediate referral for fetal echocardiography

   • Specialized cardiac imaging is indicated when standard views cannot be obtained 2
   • Fetal echocardiography is particularly important for suspected cardiac defects 2

2. Consider fetal MRI without contrast

   • MRI may be helpful when ultrasound information about an abnormality is incomplete 2
   • Ideally performed at or after 22 weeks gestation (patient is already at 25 weeks) 2

3. Genetic counseling and testing

   • Conotruncal defects may be associated with genetic syndromes
   • Amniocentesis or non-invasive prenatal testing may be considered if not already done

Potential Diagnoses to Consider

When the ductal arch cannot be visualized, several cardiac anomalies should be considered:

• Tetralogy of Fallot
• Transposition of the great arteries
• Double outlet right ventricle
• Truncus arteriosus
• Right aortic arch with aberrant left subclavian artery
• Right-sided persistent ductus arteriosus (occurs in approximately 10% of fetuses with right aortic arch) 3

Factors Affecting Visualization

Several factors may affect the ability to visualize the ductal arch:

• Maternal factors (obesity, abdominal scarring)
• Fetal position
• Decreased amniotic fluid
• Technical factors (equipment quality, sonographer experience)
• Gestational age (though at 25 weeks, visualization should be possible)

Clinical Implications and Management

The detection of cardiac anomalies in the third trimester, even after previous normal scans, is not uncommon. Studies show that:

• Approximately 1 in 300 women will have a new malformation detected at a routine third-trimester scan 4
• Some CNS and other anomalies may only become apparent in the late second or third trimester 5

If a conotruncal anomaly is confirmed:

• Delivery planning should include a pediatric cardiologist and neonatologist
• Delivery at a tertiary center with pediatric cardiac surgery capabilities may be necessary
• Close monitoring for the remainder of the pregnancy is essential

Common Pitfalls to Avoid

1. Assuming previous normal scans rule out anomalies

   • Some cardiac defects evolve or become more apparent later in pregnancy

2. Attributing non-visualization solely to technical factors

   • While technical limitations exist, persistent inability to visualize the ductal arch across multiple attempts is concerning

3. Delaying specialized referral

   • Early diagnosis allows for better pregnancy management and postnatal planning

4. Overlooking associated anomalies

   • Conotruncal defects may be associated with other cardiac and extracardiac anomalies

In summary, the inability to visualize the ductal arch on repeated attempts at 25 weeks warrants prompt referral to fetal cardiology for comprehensive evaluation, as it may indicate a significant conotruncal cardiac anomaly that could impact fetal and neonatal outcomes.