Deformation: A change from the normal size or shape of an anatomic structure due to mechanical forces that distort an otherwise normal structure. Deformations occur most often late in pregnancy or during delivery. A twin pregnancy can cause deformations due to crowding of the twins late in pregnancy. A well-known example of a deformation is molding of the head of a baby born by vaginal delivery. There are usually no significant lasting effects of a deformation. The effects are typically temporary.
A deformation is different from a malformation in timing and impact. In a malformation, the development of a structure is arrested, delayed, or misdirected early in embryonic life and the effect is permanent.