Bone Development in Babies

Babies start developing their 300 bones during the second trimester, according to

As your baby grows in utero, her bones turn from cartilage to bone, then fuse together to form an adultlike skeleton. Keeping your baby healthy and promoting good bone health will help ensure a strong skeleton for life.


The bones that make up your baby’s skeleton are alive, growing and changing, just like the rest of her body. The periosteum--a thin, dense membrane made of nerves and blood vessels that nourish the bone--comprises the outer surface of bone. Compact bone is the next layer. This smooth, hard part of bone is the part you see when you look at a skeleton. Beneath the compact bone are layers of cancellous bone.

The innermost part is comprised of bone marrow, a thick jelly that produces blood cells. As your baby grows inside of you, bones begin to replace cartilage in a process called ossification.

Time Frame

Close up of newborn baby

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Between week 13 and week 16 of gestation, your baby will begin developing bone to replace cartilage. Tiny little ribs begin to appear across his chest and his skeleton begins to take form.

Before this development, your baby’s bones were comprised of cartilage and connective tissue until they turned into bones. The bones become visible on ultrasounds in order to allow doctors to determine if there are any skeletal abnormalities.

Skull Development

Your unborn baby’s skull stays soft and flexible during the time she is inside of you. Her skull consists of five pliable bones that overlap each other in order to fit through the pelvis during birth. Two fontanels, or open spaces, are located on the baby’s skull.

These "soft spots" allow the bones of the baby’s skull to move together during the birthing process. After birth, the newborn’s head may appear strangely lopsided or conical from passing through the birth canal. This condition is temporary. As your baby gets older, usually between 6 and 18 months, the bones in his skull grow together to cover the soft spots entirely.


Close up of newborn baby

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At birth your baby’s body has about 300 bones, according to Kids Health. Some of these bones are made entirely or partly of pliable, flexible cartilage to allow her to be less prone to breaks as she is growing and learning to crawl, walk and run.

As your baby grows, her bones will eventually fuse to form the 206 bones that adults have. By the time she is 25, the entire bone development process is complete, making for a strong and light skeleton that holds the frame of her body.


Developing good eating habits when young will help babies grow to have strong, healthy bones throughout their lives. Calcium and protein both play an important role in your baby’s development of a healthy skeleton system. Under-nutrition, including insufficient protein or calcium, can impair bone development. Insufficient protein lowers the production and action of a growth factor, IGF-1, which enhances bone formation. This can result in not enough bone mass gain in babies and children as they grow.