Intrauterine development of an organ or structure that is abnormal in form, structure, or position.

A birth defect caused by a structural abnormality or a marked deviation from the average or norm; anything that is structurally unusual or irregular or contrary to a general rule; such as, a congenital defect.

There are four clinically significant medical types of anomalies: malformation, disruption, deformation, and dysplasia (abnormal in form; for example, retinal dysplasia is abnormal formation of the retina during embryonic development).

A heart abnormality, developed before birth, in which there is a hole in the partition between the left and right sides of the heart: "The septal defect is commonly known as a hole in the heart, but it varies in its effects, depending on the size and the position of the defect."

"When the hole is in the septum (dividing wall) separating the two ventricles (lower chambers of the heart), the abnormality is known as a ventricular sepal defect; when it is in the septum between the two atria (upper chambers), it is called an atrial septal defect."

"If the septal defect hole is very large, the misdirection of blood results in a greatly reduced oxygen supply to the tissues and excessive blood flow through the lungs."

"With a large septal defect, or ventricular hole, heart failure may develop six to eight weeks after birth of the child, causing breathlessness, feeding difficulties, pallor, and sweating."

"Modern surgery is considered to be so effective in dealing with large septal defects that it now enables most people who are affected to lead normal lives."