**2. Development**

The fetal skeleton starts to develop early during gestation. The appendicular and axial skeletons undergo a programmed pattern of endochondral ossification during which a cartilage template is replaced by the bone. In contrast, the calvarium and portions of the clavicle and pubis ossify via membranous ossification, whereby mesenchymal cells differentiate directly into osteoblasts [6].

Limb buds begin to develop during the fourth to fifth gestational week (GW) as clusters of mesenchymal cells covered by the ectoderm, but before the end of the seventh GW, the anatomy of the embryonic pole is difficult to observe. The upper limb development precedes the lower limbs in bud appearance, differentiation, individualization, movements, and final size. The process starts from proximal to distal, with the humerus and femur first, then ulna and radius, tibia and fibula, metacarpal and metatarsal bones, and lastly phalanges [7]. Then, the mandible, maxilla, and clavicles ossification centers appear at 8 weeks of gestation; appendicular skeleton, ilium, and scapula by 12 weeks; and metacarpals and metatarsals by 12–16 GW [6].
