Soft bone development begins during pregnancy at 7 weeks, and is most active during the third trimester. After birth, the bones become hardened through a process called “ossification” . Until then, bones are mostly made of cartilage at birth, which is soft and rubbery.
In the long bones of the arms and legs, specific areas near the joints contain the most active areas of ossification. These areas are called growth plates. The bone cells that live there convert the calcium, phosphorus and other minerals taken from the baby’s diet into a cement-like substance called “bone mineral” (also called “matrix”).
Bone mineral is built-up a layer at a time across the width of the bone, making it gradually elongates. In the leg bones, this contributes to increasing height. This is most active in the first 5 years, and sometimes causes “growing pains”. Bone mineral is important because it enables the body to hold itself up as the baby gets older, especially after the first year, when they learn to walk.
At the same time, muscles get longer and stronger and perform physical functions in coordination with the skeleton. This also stimulates bone growth.