This one was harder than I thought. A fetal skeleton should have all the same bones as an adult skeleton, right?
Well no, actually, and it depends. It depends on when you check. A human fetus has no bones at all until the 7th week. Two bones come in during the 7th week. The femur, which is not surprising as it’s the largest, hardest single bone in the adult body. The second bone is the clavicle, also known as the collar bone, an oddly shaped little bone that holds up the neck. It’s small, but think of how important it’s job is.
The rest of the bones develop from the 8th to 15th week (2nd to 4th month). So by the 5th month a fetal skeleton is complete. Well almost complete. As we all know the teeth come in after birth, as does the knee cap (patella) which grows in between the 3rd and 6th year. That’s why baby legs have such a smooth taper from thigh to ankle – no knobby knee in the middle.
The last bone that the fetus grows is the hyoid bone, also known as the lingual bone or voice box. It’s the only bone in the human body not rooted against another bone, and it’s the one that makes articulate speech possible. In animals the hyoid bone is simply another part of the neck, not the floating wonder it is in humans. In fetal development the hyoid bone doesn’t grow until the 36th week, the 9th month. Then a baby is ready to be born.
A complete chart of fetal bones and their developments is available here