Babies are born with around 300 bones, but some of these will fuse together with age. By adulthood, they’ll have just over 200 bones.

It may be difficult to imagine when looking at a tiny newborn baby, but that infant has around 300 bones — and those bones are growing and changing shape every day.

Adults, on the other hand, have 206 bones, which make up about 15 percent of their body weight.

Wait — did we really just say that babies have nearly 100 more bones than adults? How is that possible?

Well, even though bones appear to be tough and rigid, they’re actually made up of living tissue and calcium that’s always being built up and discarded throughout your life.

Let’s take a closer look at how this explains the discrepancy between a baby and you.

Most bones are made of several layers of tissue:

  • periosteum: the thick membrane on the outer surface of the bone
  • compact bone: the smooth, hard layer that’s seen in the bones of a skeleton
  • cancellous: sponge-like tissue within the compact bone
  • bone marrow: the jelly-like core of the bones that makes blood cells.

The process of bone development is called ossification. It actually begins around the eighth week of embryonic development — pretty incredible!

Even so, at birth, many of your baby’s bones are made entirely of cartilage, a type of connective tissue that is tough, but flexible. Some of your little one’s bones are partly made of cartilage to help keep baby nice and, well, malleable.

That flexibility is necessary so growing babies can curl up in the confined space of the womb before birth. It also makes it easier for mom and baby when it’s time for baby to make the exciting journey through the birth canal during delivery.

As your baby grows into childhood, much of that cartilage will be replaced by actual bone. But something else happens, which explains why 300 bones at birth become 206 bones by adulthood.

Many of your baby’s bones will fuse together, which means the actual number of bones will decrease. The space that separates the ends of two bones that eventually fuse is also cartilage, like the tissue you have in the tip of your nose.

The fusing of bones occurs throughout the body. You may notice that there are one or more soft spaces in between the bones in your baby’s skull. These “soft spots” may even freak you out a bit, but they’re perfectly normal. They’re called fontanelles, and they’ll eventually close as bones grow together.

Replacing cartilage with fused bone begins when tiny blood vessels — called capillaries — deliver nutrient-rich blood to osteoblasts, the cells that form bones. Osteoblasts create bone that covers cartilage at first and then ultimately replaces it.

Then, bone growth in children occurs at the ends of many bones, which have growth plates. The growing tissue in each plate determines the final size and shape of the bone. When a person stops growing, the growth plates close.

Growth plates are weaker than other parts of your child’s skeleton, and are therefore more susceptible to fractures and other injuries. This is why a fall off a bicycle may land your kid in a cast, whereas you can take a similar fall and just have a bruise — perhaps on your body as well as your ego.

What’s the role of calcium in all this?

Calcium is the mineral essential to the formation of new bone tissue. It’s found in both breast milk and formula. And if your child is resistant to eating their leafy greens later on, remind them that the calcium found in these vegetables (as well as in dairy products) helps them grow.

By early adulthood, the fusing of bones and bone growth have stopped. Adult bones are very strong, but light. And surely now that you have your 206 bones, you’re all set, right?

Well, not exactly. While they appear to be solid and unchanging, bones are constantly going through a process called remodeling. (But it’s true that the number of bones you have won’t typically change after this point.)

Remodeling involves the formation of new bone tissue and the breakdown of older bone into calcium and other minerals, which are released into the bloodstream. This process is known as resorption, and it’s a perfectly normal and healthy part of bone function — in fact, it happens throughout life. But in children, new bone formation outpaces resorption.

There are some things that can speed up bone loss. These include:

  • the hormonal changes associated with menopause
  • excessive alcohol consumption
  • advancing age

The most common condition affecting bone loss is osteoporosis, which causes bones to lose some of their density and become more vulnerable to fractures.

The framework of bones and joints in the human body is both complex and fascinating — just like you. Bones fit together like a massive puzzle, and rely on a variety of muscles to move at joints from the neck and jaw down to the toes.

Bone facts

  • The part of the body that contains the most bones is the hand. It’s made up of a whopping 27 individual bones.
  • Most red and white blood cells in the body are created in bone marrow.
  • The femur, located in the thigh, is the longest bone in the body.
  • The stapes, a stirrup-shaped bone located deep in the ear, is the body’s smallest bone.
  • Bones store about 99 percent of the calcium in your body and are composed of about 25 percent water.
  • Your skeleton completely replaces itself every 10 years or so through remodeling. It’s kind of like remodeling your kitchen, except the new one looks eerily similar to the old one.
  • There are two types of bone material: cortical, the hard kind that you think of when you picture a skeleton, and trabecular, which is softer and spongier and often found inside large bones.
  • Some bones are designed to withstand two to three times your body weight in force.
  • Cartilage tissue has no regular blood supply and doesn’t renew, so cartilage injuries are permanent. Fortunately, they’re also less common.
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The process of bone growth and fusion in children is a remarkable one. And to make sure your child’s bones stay healthy for years to come, it’s important to pass along some important lessons. Among them:

  • Get enough calcium in your child’s diet (and yours, too). The body doesn’t make calcium, so all the calcium you need has to be consumed in food or supplements. Healthy calcium-rich foods include low-fat dairy products (milk, cheese, yogurt), seeds, almonds, white beans, and leafy greens, such as spinach and collard greens.
  • Make weight-bearing exercises, such as walking or weight lifting, part of your usual workout routine or a fun family activity. Exercises that safely test your bones and muscles can help promote bone health throughout adulthood — but it’s never too early to start thinking about this!
  • Make sure you get enough vitamin D in your diet or through supplements. Vitamin D helps your body absorb calcium. Getting enough protein is also important for long-term bone and muscle strength. If your child surprises you by declaring their vegetarianism early, make sure they know good sources of protein aside from meat. (And always speak with a pediatrician about changes in diet.)