Congenital malformations may be the result of disturbances of development or of disease during fetal life.

There may be complete absence, as in acardiac monsters. Imperfect septa between the cavities is the most common defect. There may be no septum and a simple heart of two cavities, like that of a fish, is formed. If the ventricular septum is absent there are two auricles and one ventricle, the reptilian type. The auricular septum is often incompletely closed, giving rise to a patulous foramen ovale. The heart may be completely reversed, lying on the right side of the body with the aorta coming from the right ventricle and the other vessels correspondingly shifted. Is known as dextrocardia.

The arterial openings may be much smaller than normal, particularly the pulmonary. When the latter occurs there is marked cyanosis. If the stenosis is of a high grade the pulmonary circulation may be maintained by a persistent ductus arteriosus. Gives rise to hypertrophy of the right ventricle.

The valves may vary in number and also in length.