The cause of a ventricular septal defect is congenital heart disease, which is abnormal development of the heart in the womb.

The ventricular septum is the wall that separates the lower chambers of the heart, called the ventricles. A ventricular septal defect creates a disturbance in blood flow through the heart. Such a defect allows blood to flow from the left ventricle to the right ventricle. This can damage the right ventricle and damage the lungs.