What is the difference between a malformation and a deformity present at birth?

In biology, the terms malformation and deformity represent distinct types of structural abnormalities, though they are often grouped under the broader category of congenital disorders.

Malformations

A malformation is defined as an irregular or abnormal structural development that typically occurs during the early embryonic stages. These anomalies arise from intrinsic errors in the developmental process, which can be triggered by mutant genes, environmental factors, infections, or a combination of these influences. The study of these structural deviations is known as teratology.1

Deformities

In contrast, a deformity is characterized as a secondary bending or change in the shape of a structure that was otherwise developing normally. These are often caused by extrinsic mechanical forces, such as the pressure exerted by the uterine wall when there is a lack of amniotic fluid. While some deformities result from external pressure, others may stem from intrinsic issues like muscle or nerve dysfunction that lead to secondary joint or limb changes.2

In summary, while a malformation is an error in the initial formation of a structure, a deformity is a secondary alteration of a structure that had begun to form correctly.12