The defect may be caused by an abnormality in a gene, a group of genes, or a chromosome. Deformation refers to intrinsic or extrinsic biomechanical forces acting on the fetus to cause a structural abnormality. These are usually benign and, with the constraints removed, can be corrected. Realize that a primary malformation can lead to a secondary deformation. The risk of recurrence will depend upon the initiating event.
Disruption refers to the breakdown of a normally formed tissue or structure.
Limbs can be amputated by an amniotic band. When a cascade of effects can be shown to occur as the result of a primary defect the term sequence has been used. An example is the oligohydramnios sequence just described which can lead to dilated ureters and enlarged cystic kidneys. Summary This section is not meant to be an exhaustive review of congenital malformations. Rather, it focuses on surface physical features that should be apparent to the careful observer.