I believe it's made out of cartilage. Just like our knees and noses are made out of. The mother would need to consume more calcium to aid the fetus in growing strong bones for the baby. <span />
This cartilage forms the initial skeleton of the fetus; right after three months of development. This cartilage is the precursor to bones that will develop from the endochondral ossification process.
During endochondral bone development, hyaline cartilage functions as an epiphyseal growth plate and this plate remains functional as long as the bone is growing in length. In the adult's long bone, hyaline cartilage is present only on the joint surface. In adults, it is also present as a skeletal unit in the trachea, bronchi, larynx, nose and extremities of the ribs (costal cartilages).
