This is the ultrasound scan done on a woman who presented for the first time during the 3rd trimester. She had no significant past medical history.

At 23 week, the four-chamber view of the heart demonstrated distortion of the contour of the right ventricular wall, characterized by a broad-based out-pouching defect. The defect encompassed the proximal two-thirds of the right ventricular wall from the level of the moderator band to the right atrial-ventricular septum. The wall of the lesion was thin and irregular. Color-flow pulsed Doppler interrogation demonstrated unrestrictive filling of the defect during diastole and subsequent emptying with systole. The wall contracted during right ventricular systole, but appeared slightly hypocontractile. Tricuspid valve regurgitation was present. The remainder of the cardiac anatomy was normal with the great vessels arising from the appropriate chambers. Normal cardiac rhythm was demonstrated by M-mode Doppler. The patient was followed with serial ultrasound surveillance for evidence of cardiac failure, thrombus formation, pericardial effusion, or progressive enlargement of the defect. The fetus transiently developed an ectopic rhythm in the form of premature atrial contractions, which spontaneously resolved. 

An appropriate-for-gestational age sized male infant was born at 39 weeks by cesarean section. Apgars were 9 and 9. Postnatal echo demonstrated the lesion, tricuspid valve regurgitation, small patent ductus arteriosis (which subsequently closed), and qualitatively normal right ventricular function. There was no pericardial effusion. At birth, the diverticulum measured 2.49 cm in its longest superior to inferior dimension, and protruded laterally 1.1 cms. The infant, now one year old, has remained asymptomatic, and is being conservatively managed with serial follow-up.

In having the scan reviewed by several international fetal echocardiologists, this is the consensus that has emerged that this is what is typically described as a diverticulum. The reasons include a neck more narrow then the width of the lesion, a thick wall (thus more likely to have all cardiac layers then an aneurysm), lack of dysmotility and good post-natal outcome. However this diagnosis is contradicted by the "dictionary" definition that a diverticulum occurs in hollow viscus like bowel and bladder while aneurysm occur in vascular structure. That definition appears too morphologic to me and that is why I accepted Diverticulum as the correct answer over Aneurysm. For an example of aneursm see:Aneurysm, left ventricle