Category: Heart

Tetralogy of Fallot (TOF) and Double Outlet Right Ventricle (DORV) can look very similar on fetal ultrasound.

Both involve:

• VSD
• Aortic override
• Abnormal outflow tract alignment

So how do we differentiate them?

1️⃣ The Key Question

👉 Where does the aorta arise from?

That is the core difference.

TOF (Tetralogy of Fallot)

✔ Aorta overrides the VSD

✔ But still primarily connected to the left ventricle

✔ Pulmonary stenosis present

On 4-chamber view:

• VSD visible
• Overriding aorta partially over septum

On outflow tract view:

• Pulmonary artery smaller than aorta

👉 Aorta originates mainly from LV (with override)

DORV (Double Outlet Right Ventricle)

✔ Both aorta and pulmonary artery arise predominantly from the right ventricle

✔ Large VSD is required for LV output

Key point:

👉 Aorta arises entirely or mostly from RV

On imaging:

• Both great vessels aligned over RV
• VSD provides LV connection

2️⃣ Why They Look Similar

Because both show:

• Overriding aorta
• VSD
• Abnormal outflow relationship

But the degree of override matters.

TOF → partial override

DORV → near complete RV origin

3️⃣ Practical Ultrasound Clues

✔ Assess the relationship of great arteries to ventricles

✔ Trace the outflow tracts carefully

✔ Look at subaortic conus presence

✔ Evaluate pulmonary stenosis severity

If pulmonary stenosis is severe and aorta is mildly overriding → think TOF.

If both great arteries sit mostly over RV → think DORV.

4️⃣ Why It Matters

Because prognosis and surgical planning differ.

TOF:

• Well-defined surgical repair
• Often isolated

DORV:

• Multiple anatomical subtypes
• Surgical strategy depends on VSD location

Clinical Tip

When unsure:

👉 Ask: Does the LV have a direct connection to the aorta?

If yes → more likely TOF

If no → think DORV

When a ventricular septal defect (VSD) is found on prenatal ultrasound,

one of the first questions parents ask is:

“Will it close on its own?”

The answer is:

👉 Yes, sometimes it does — even before birth.

But it depends on the type and size.

1️⃣ Small Muscular VSD

This is the type most likely to close spontaneously.

• Located in the muscular part of the septum
• Often small (1–2 mm)
• May become smaller as the fetal heart grows
• Can close before birth or within the first year of life

✔ Prognosis is usually excellent

✔ No major complications if isolated

2️⃣ Perimembranous VSD

This is the most common type.

• Located near the aortic valve
• Small defects may close
• Larger ones are less likely to close spontaneously

⚠ Needs follow-up because of possible:

• Aortic valve prolapse
• Aortic regurgitation

3️⃣ Inlet or Outlet VSD

These types are less likely to close before birth.

• Often associated with other structural findings
• May require postnatal cardiology follow-up
• Sometimes surgical repair is needed

Does Closure Happen in the Womb?

Yes — but not always.

Closure can occur because:

• The septum continues to grow
• Tissue gradually covers the defect
• Hemodynamic forces change as the heart matures

However, some VSDs remain stable throughout pregnancy.

Important Perspective

A small isolated VSD:

• Does not usually affect fetal growth
• Does not cause fetal heart failure
• Does not require early delivery

Most babies with small isolated VSDs are born healthy.

When Should Parents Worry?

Concern increases when:

• The VSD is large
• There are additional heart defects
• There are extracardiac anomalies
• Chromosomal risk is elevated

In those cases, further evaluation is recommended.

Bottom Line

Yes — some VSDs close before birth.

Especially small muscular defects.

The key factors are:

👉 Location

👉 Size

👉 Associated findings

Not just the presence of a “hole.”