"Chiken Egg with a Contriction Band "

Mechanical factors can deform an embryo or fetus and cause malformations such as amputations and deformations, a "club foot" for instance. In humans, rupture of the amniotic sac may cause constriction bands called "amniotic bands" which have been shown to be associated with limb and digit amputations and other malformations. Amniotic bands are an example of exogenours or extrinsic causes of "birth defects", while malformations due to gene mutations and chromosomal aberrations are considered to be due to "internal or constitutional" causes (etiology).