How Many Bones Are Babies Born With? Why 300 Become 206 in Adults
Babies are born with about 300 bones and adults have 206. How do bones fuse and why is the baby skull soft?
Short answer: Babies are born with roughly 270 to 300 bones or unossified cartilage pieces, while adults have 206 bones. No bone disappears: during growth many small bones and cartilage pieces fuse together. This is clear in the skull, the tailbone area of the spine and the hip bones, and continues after puberty and, in some places, into the twenties.
Dr Ahmet Şadi Kılınç, Orthopaedics and Traumatology Specialist, Bahçeşehir, Istanbul
“Do babies have more bones?” is a question that interests parents and students alike. The short answer is yes: babies are born with more bone pieces than adults. In this article I explain why, how bones fuse, and why a baby’s skeleton is flexible.
I also explain it in a short video, in Turkish: watch on YouTube.
Why are babies born with more bones?
At birth, much of the skeleton is not yet bone but cartilage. Cartilage is flexible, which helps the baby grow in the tight space of the womb and pass through the birth canal. As the baby grows, cartilage turns into bone from ossification centres. Some bones start as several pieces that join into a single bone over the years.
This is why the number varies between 270 and 300 depending on what is counted as a “bone”. The accepted adult number is 206.
Where do bones fuse?
- Spine (sacrum): The 5 separate lower vertebrae form a single bone, the sacrum, in adults. The tailbone also forms from a few small pieces.
- Hip bone: The three bones that form the hip bone (ilium, ischium, pubis) are separated by cartilage in childhood and fuse towards puberty.
- Ends of long bones: The ossification centres at the ends of arm and leg bones (epiphyses) join the shaft when the growth plate closes, usually around 14 to 16 years in girls and 16 to 18 in boys.
- Skull: At birth the skull is made of plates that are not yet fused.
Why is the soft spot on a baby’s head important?
The soft spots on a baby’s head are called fontanelles. The gaps between the skull plates serve two purposes:
- The skull can change shape slightly during birth, which eases passage.
- The skull can expand with the brain as it grows quickly in the first years.
The front fontanelle closes at about 12 to 18 months in most babies, and the back one usually in the first 2 to 3 months. If a fontanelle is very sunken, very bulging or stays open much longer than expected, speak to your paediatrician.
The skeleton is living tissue
Bone is not just scaffolding; it is living tissue that is constantly renewed. Bone cells break down old tissue and build new. This renewal allows growth in children and fracture healing in adults. Peak bone mass is reached around ages 25 to 30, followed by a slow decline. Investing in bone health in childhood and adolescence lowers the risk of osteoporosis later in life.
What matters for bone development in children?
- Adequate, balanced nutrition (calcium, protein)
- Sunlight and, if your doctor advises, vitamin D
- Regular weight-bearing activity such as running and jumping
- Enough sleep
- Regular growth checks
When to see a specialist
Have your child assessed by an orthopaedic or paediatric specialist if you notice:
- Uneven hip or thigh skin folds in a baby, or suspected hip dislocation
- Marked bowing of the legs inwards or outwards, or one leg shorter
- Marked delay in walking, or a limp
- A curve in the back, or uneven shoulders or hips
- Frequent fractures or bone pain
For more on bone and joint health in children, see our Paediatric Orthopaedics page. For other building blocks of bone health, read our article on calcium and bone health.
This article is for general information and does not replace an examination.
Frequently asked questions
How many bones are babies born with?
About 270 to 300 bones or unossified cartilage pieces. The number varies between sources depending on what is counted as a bone.
How many bones does an adult have?
206.
Where do a baby's bones go?
None disappear. During growth many small bones and cartilage pieces fuse into fewer, larger bones.
Why is a baby's skull soft?
The skull plates are not yet fused at birth. This flexibility eases birth and lets the brain grow in the first years.
When do the fontanelles close?
The back one usually in the first 2 to 3 months, the front one at about 12 to 18 months.
The information on this page is general and does not replace an examination. An examination is needed to decide which treatment suits you.