Why babies have more bones than adults

Here is a fact that surprises almost everyone: a newborn baby has more bones than its parents. Adults are usually counted as having 206 bones, while babies start with somewhere around 270 to 300 separate bony pieces. Nothing goes missing along the way. Instead, many small pieces slowly grow together, a process that starts before birth and is not fully finished until early adulthood.

The short answer

Babies have more bones because their skeletons are built in pieces. Many adult bones begin as two, three or more separate parts, each growing from its own ossification center, a spot where bone tissue first starts to form. In a newborn, many of those parts are still separated by flexible cartilage or fibrous tissue.

As a child grows, the pieces enlarge, the cartilage between them turns to bone, and neighboring parts fuse. Every fusion reduces the count by one. By the time the skeleton matures, the standard total has settled at 206. You can see where every adult bone ends up in our complete list of the 206 bones.

Why a flexible skeleton helps

A skeleton made of many pieces is not a design flaw. It solves two real problems.

• Birth. The bones of a baby's skull are not yet joined, so they can shift and overlap slightly as the head passes through the birth canal. This is why some newborns have a temporarily elongated head shape that rounds out in the following days.
• Growth. The brain grows very quickly in the first years of life. Open seams between skull bones let the skull expand to keep pace. In the limbs, cartilage zones near the ends of long bones allow bones to lengthen for well over a decade.

How bone forms: two pathways

Bone formation is called ossification. It begins early in fetal life, and the skeleton builds bone in two different ways.

Intramembranous ossification

In this pathway, bone forms directly inside sheets of primitive connective tissue, without a cartilage model first. Bone-building cells called osteoblasts lay down new matrix, which hardens with minerals and traps some of the cells inside, where they mature into osteocytes. This route produces most of the flat bones of the skull, the facial bones and the clavicles (collarbones).

Endochondral ossification

Most of the skeleton takes a slower route. The body first builds a small model of the future bone out of cartilage, then gradually replaces that cartilage with bone. This is how the long bones of the arms and legs, the vertebrae, the ribs and the base of the skull form.

In a typical long bone, a primary ossification center appears in the shaft during the early months of fetal development. After birth, secondary ossification centers appear in the rounded ends. Between the shaft and each end, a plate of cartilage remains for years. That plate is the growth plate.

Fontanelles and sutures: the baby skull

A newborn's skull vault is made of separate plates of bone joined by flexible fibrous seams called sutures. Where several plates meet, there are wider membrane-covered gaps known as fontanelles, the soft spots parents notice on top of a baby's head.

Anatomy references describe six fontanelles in all, but two are the ones people can usually feel:

• The anterior fontanelle on top of the head, toward the front, is the largest. It typically closes somewhere between about 7 and 24 months; different references give slightly different ranges, and healthy babies vary.
• The posterior fontanelle at the back of the head is smaller and usually closes within the first one to two months. In some babies it is already nearly closed at birth.

The smaller fontanelles at the sides of the skull generally close during the first year or so. The skull bones themselves stay separate for many months, and the sutures between them remain as joints into adulthood, gradually stiffening over the decades.

Which bones fuse together?

Fusion happens all over the body. A few well-known examples show how the count drops from roughly 270–300 to 206.

Growth plates: when growing stops

Growth plates, also called epiphyseal plates or physes, are discs of cartilage near the ends of long bones. Cartilage cells in the plate multiply and are then replaced by bone on the shaft side, which pushes the end of the bone farther out. This is how children get taller.

When growth is complete, the cartilage stops dividing and is replaced entirely by bone. The plate closes and leaves only a faint line visible on an X-ray. After that, a bone can no longer grow in length, though it keeps remodeling and changing in density for life.

• Timing varies by person and by bone. Most growth plates in the arms and legs close sometime in the mid-to-late teens.
• Girls usually finish earlier than boys, often by a year or two, because puberty tends to start earlier.
• A few sites mature later. The inner end of the collarbone is widely regarded as one of the last places to finish ossifying, with fusion continuing into the 20s. The sacrum also completes its fusion in the mid-20s or later.

Because open growth plates are weaker than the ligaments and tendons around them, they are a common site of injury in active children and teens. A suspected growth plate injury should always be checked by a clinician, since proper treatment helps the bone keep growing normally. Our guide to broken bone healing covers fractures more broadly.

Building strong bones while they grow

Childhood and the teen years are when the skeleton builds most of its lifetime bone mass. The stronger the skeleton is by early adulthood, the more reserve it has later in life. A few basics support that process:

• Enough calcium and vitamin D from food, sunlight and, when recommended, supplements. Our calcium and vitamin D guide and the calcium calculator can help you estimate needs by age.
• Weight-bearing and impact activity such as running, jumping, dancing and sports, which signals bones to grow denser. See exercises for strong bones or try the bone exercise planner.
• Overall nutrition and sleep, since growth depends on adequate energy, protein and hormones that are released largely during rest.

Key takeaways

• Babies have roughly 270 to 300 bony pieces; adults are counted as having 206 bones.
• The count drops because separate pieces fuse, not because bones disappear.
• Bone forms either directly in membranes (most skull bones, clavicles) or by replacing a cartilage model (most of the rest).
• Soft spots on the skull close in the first months to about two years.
• Most limb growth plates close in the teens, while a few sites, such as the sacrum and the inner collarbone, keep maturing into the 20s.

Curious how 206 became the number at the heart of this site? Read what 206 means, then put your knowledge to work in the bone explorer.

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