Actual birth defect
The term "birth defect" is usually applied to unusual physical features.
There are 2 causes. 1) a problem not due to physical forces outside of the fetus, termed malformation. 2) a problem due to physical forces which have acted upon a previously normal fetus, called deformations. The squashed-appearing face
of an infant with prolonged lack of adequate amniotic fluid
is an example of 2).