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Fetal development occurs in three separate and distinct phases. The first is the pre-differentiation period in weeks 1 and 2, the second is the embryonic period that occurs during weeks 3 through 8, and the last is the fetal period from week 9 onward. During the embryonic period, differentiation of the upper limb morphology begins with the upper limb bud around the fifth week. The collection of cells located at the distal ridge of the limb bud, defined as the apical ectodermal ridge, mediates the differentiation and maturation process for the upper extremity. Mesenchymal condensation leads to the formation of cartilaginous analogs of the shoulder, the arm, the forearm, and ultimately the hand. These analogs ossify into bones around week 6 of gestation. The primary centers of ossification begin to form long bones as early as the 12th week. The sequential formation of the cavities in the joints, the condensation of the ligaments, and differentiation of the muscles begins first in the shoulder girdle and proceeds distally to the hand during the 6- to 8-week time frame. After 9 weeks, the bones, joints, ligaments, and muscles undergo further maturation.[1]

During the embryonic period in gestation, limb territories are first determined in the right positions along the cephalocaudal axis. The limb bud grows out from the body as a mesenchymal cell mass covered by ectoderm. The identity, position and cell masses depend on the expression of Hox genes.[9] The first category of limb malformations is a failure of formation, which is arrestment of development of a particular portion of the limb. Transverse deficiencies include all congenital amputations, such as shoulder, arm, forearm, carpals, metacarpals, and phalangeal clinical entities. Longitudinal deficiencies include all the skeletal limb deficiencies not included in the transverse, such as phocomelias. These may range from partial to complete absence. The second category of limb malformation is a failure of differentiation, which is defined as the basic unit of the limb has developed, but its final form is incompletely developed. An example of this is syndactyly or incomplete separation of the adjacent digits. The third category is duplication, which results from an insult to the apical ectodermal ridge during development. An example of this is polydactyly. The fourth category is known as overgrowth, which is a nonhereditary congenital enlargement. This commonly presents in the digits. The fifth category is denoted as hypoplasia or undergrowth, where there is incomplete growth during the fetal period. The last category is included but is considered a separate entity. It is congenital constriction of a limb due to bands, which leads to subsequent loss of the part.[1]