What is the difference between a malformation and a deformity?

In the study of congenital disorders, the distinction between a malformation and a deformity lies primarily in the timing and nature of the developmental disruption.

• Malformation: These are structural abnormalities that arise during the early stages of development, specifically during blastogenesis and organogenesis. They represent an irregular or abnormal structural development often caused by mutant genes, environmental factors, infections, or drugs that interfere with the normal course of embryonic growth. Examples include cleft palate, spina bifida, or the absence of limbs.1234
• Deformity: These are defined as a secondary bending or change of shape. Unlike malformations, which involve an error in the initial formation of a structure, deformities often result from extrinsic pressure—such as a lack of amniotic fluid (oligohydramnios)—that affects a fetus that was otherwise forming normally. This pressure from the uterine wall can lead to conditions like flattened facial features or joint fixations, such as clubbed hands and feet.5

While some deformities may stem from intrinsic issues like nerve or muscle dysfunction, they are fundamentally characterized as alterations to the shape of previously formed structures rather than primary errors in organogenesis.5