Which is the most common form of cyanotic congenital heart disease in the fetus?

• A. Tetralogy of Fallot
• B. Transposition of the great arteries
• C. Hypoplastic left heart syndrome
• D. Tricuspid atresia

Answer: (A)

The main idea here is that among cyanotic congenital heart diseases seen before birth, Tetralogy of Fallot is the most commonly identified. In the fetus, oxygenation depends on mixing of blood through the foramen ovale and ductus arteriosus. Tetralogy of Fallot causes cyanosis because of outflow tract obstruction to the pulmonary artery combined with a VSD and an overriding aorta. The pulmonary stenosis limits flow to the lungs and promotes right-to-left shunting, so more desaturated blood reaches the systemic circulation even before birth. This prevalence and the way the lesion affects fetal circulation make it the most commonly detected cyanotic CHD in utero. The other lesions listed are less common in the fetal period, though they are indeed cyanotic postnatally.