Prenatal Development of the Skeletal System
Prenatal Development is often divided into three parts, called trimesters.
http://www.mayoclinic.com/health/medical/IM04611
First Trimester (Weeks 1-12)
- The embryonic period starts the fifth week after conception. The embryo of the baby forms into three layers, called the ectoderm, mesoderm, and endoderm. The mesoderm is the middle layer that begins to form, and is the foundation of the baby's bones.
- The baby continues to develop, and by week nine, bones in the arms and legs begin to develop and grow.
http://www.mayoclinic.com/health/medical/IM04072
Second Trimester (Weeks 12-27)
- By week thirteen, tissue that will become bone develops around the baby's head.
- By week fifteen, the baby's skeleton rapidly develops bones and continues to grow. The skull becomes more prominent around this time as well.
http://www.mayoclinic.com/health/medical/IM04608
Third Trimester (Weeks 28-40)
- By this point, most of the bones in the baby have been laid out and continue to develop until they are full-term.
- By the end of the third trimester, babies have 300 bones, which will eventually fuse to become 260 bones.
- These bones form from cartilage, a flexible substance, and eventually turn into bone through the process of ossification.
