Why Do Babies Have More Bones than Adults?

An overview of how many bones babies are born with

Did you know that at birth a baby has more bones in its body than an adult? Babies are born with about 270 bones, some of which will eventually fuse to form the 206–213 bones that they will have as adults.

Learn about how a baby's bones change over time.

A light-skinned baby's head is cradled in a person's palms

Science Photo Library - Ian Hooton / Getty Images

Bones and What They’re Made Of

A baby’s bones consist of more cartilage than solid bone. Cartilage is more flexible than bone and is able to resist compression and provide support and flexibility.

Unlike bone, cartilage does not contain calcium in its matrix, the fibers and other substances that make up cartilage. Instead, cartilage contains the chemical chondroitin, which keeps it flexible and elastic. Cartilage also does not contain blood vessels or nerves. Instead, blood flow and pain sensations come from the surrounding structures.

Over time, much of the cartilage in a child's body hardens into bone—a process called ossification. Ossification actually begins in utero around the sixth or seventh week of gestation and continues through a person’s mid-20s.

There are two types of bone ossification. They are called intramembranous and endochondral ossification, and they are responsible for the following bone formations:

  • Intramembranous: Forms the flat bones of the skull, clavicle, and cranium
  • Endochondral: Forms the axial skeleton and long bones

The Bones That Babies Are Born With

Bones in the human body fall into three categories: long, short, and flat. A baby is born with bones and cartilage that will eventually ossify into these stronger bones.

Long bones evolve through endochondral ossification. Long bones are typically tubular, and they are longer than they are wide. The arms and legs are made up primarily of long bones.

Short bones also evolve through endochondral ossification, but they are not structurally similar to long bones. Instead, they take on distinct shapes. For example, the bones of the ankles, wrists, heels, and kneecaps are all short bones.

Flat bones evolve through intramembranous ossification. These bones have platelike shapes. The skull bones, the breastbone, shoulder blades, ribs, and hip bones are examples of flat bones.

The Ways That Bones Change as Babies Grow

A baby’s bones change from softer, flexible cartilage to harder, firmer bones over time. The process takes place from before birth until a person is an adult. 

The Skull

Several bones make up the skull—two frontal bones, two parietal bones, and one occipital bone. Over time, these bones will fuse, but it is initially important that they remain flexible in order to go through the birth process and for brain growth. 

Fontanelles are the spaces between the bones. There is one in the front (known as the soft spot) and one in the back of the head. They are covered by membranes that protect the underlying tissues and the brain.

The anterior fontanelle (soft spot) remains soft until a child is about 2 years of age. The posterior fontanelle usually closes by about 3 months of age.

Craniosynostosis

Craniosynostosis is a congenital disability (present from birth) in which the skull bones fuse too early. When this happens, as a baby’s brain grows, its skull can become misshapen. It occurs in one in 2,500 births in the United States. Oftentimes, surgery allows space for the brain to grow properly.

The Spine

A healthy adult’s spine has several natural curves—cervical (at the neck), thoracic (mid-back), and lumbar (at the base). The curves allow for proper mobility as we walk, bend, twist, and do everyday movements. However, our spines do not start out with these curves.

A baby’s spine begins in simple a C-shape. The thoracic curve begins to develop in utero, while the secondary cervical and lumbar curves do not develop until infancy, when a baby begins to lift its head, sit, crawl, and eventually stand and walk.

Spines can also curve abnormally. Kyphosis and lordosis are spine abnormalities that occur when a child’s front-to-back spinal curvature is too large. Scoliosis is a spine condition in which the spine is curved from side to side.

Scoliosis can be congenital or neuromuscular (a consequence of a muscular or neurological condition). The treatment depends on the severity of the condition and can include observation, bracing, or surgery.

Spina bifida is a neural tube defect that affects the spine. It occurs when the neural tube (which becomes the brain and spinal cord) does not close all the way during early embryonic development. When this happens, the bones that are supposed to protect the spinal cord do not form and close, which damages the spinal cord and surrounding nerves.

Spina bifida can result in physical and intellectual disabilities, but not everyone with spina bifida will have disabilities. The treatment depends on the severity of the condition and may include surgery or mobility aids, like wheelchairs, braces, crutches, and walkers.

The causes of spina bifida are not completely understood. However, one known way to reduce the risk of the condition is to take folic acid supplements if you are trying to conceive and while you are pregnant.

Arms and Legs

Ossification of the long bones actually begins in the fetal stage. Ossification of the limbs is not complete until the late teens to mid-20s.

Long bones in children are divided into four regions: diaphysis, metaphysis, physis, and epiphysis.

  • Diaphysis: The shaft, where primary ossification occurs
  • Metaphysis: Where the bone flares
  • Physis: Also called the growth plate
  • Epiphysis: Secondary ossification center

Only the metaphysis and diaphysis are present in adults. The epiphysis is mostly cartilage in infants. Over time, it gradually ossifies until it becomes almost completely replaced by bone in a person’s late teens.

Children’s bones are more flexible and have a thicker covering, making them more able to absorb shock and resist fractures. At the same time, kids are more prone to some kinds of fractures in areas that are no longer present in adults, such as growth plates.

If growth plates (tissue near the ends of bones) do not heal properly, bones may grow crooked or more slowly. For this reason, a doctor will want to monitor a child’s bone for a year or more after a fracture.

Rare Bone Disorders

Achondroplasia is a rare genetic bone disorder that prevents cartilage from being made into bone. It causes a child to develop with short arms, legs, and a larger head. The condition is a form of dwarfism. People with achondroplasia have typical intelligence and life spans.

Hypochondrogenesis is a rare genetic condition that causes a fetus to develop shorter limbs, a small body, and abnormal ossification of the spine and pelvis. The condition is usually fatal before birth or shortly after.

Other Facts About Bones

Bones are a fascinating part of the human body. A child's bones are constantly changing, with bones developing into new, different bones all the time.

Here are some other interesting facts about bones that you might not know:

  • For most people, bone mass peaks in their late 20s.
  • Osteoporosis, characterized by a decrease in bone mass and density and an increase in bone spaces, occurs most often in older people but can also occur in young adults.
  • Healthy bone behaviors in youth, like getting enough calcium and physical activity, can decrease the risk of developing osteoporosis later in life.
  • Exercise makes bones stronger, especially weight-bearing activities and resistance exercise. 
  • Smoking is harmful to bone tissue.
  • Kids with asthma need to be extra mindful about caring for their bones because the corticosteroid medications that are used to treat asthma can decrease calcium absorption.

Frequently Asked Questions

How can I keep my baby's bones healthy?

Being a positive role model is a great way to establish good habits in your kids. Good nutrition and physical activity are the two factors that influence bone growth and health. For the first year, your child will receive adequate nutrition through breast milk or formula. After that, make sure your child gets plenty of foods that are rich in calcium and vitamin D.

When will my baby's bones fuse?

The process of bones fusing is called ossification. Ossification doesn’t happen all at once. Instead, it is a process that begins in utero and continues until the skeleton is mature—usually in a person’s 20s.

When will my baby's skull fuse?

A baby’s skull is made up of five bony plates. The spaces between these bones are called the fontanelles.

There is one fontanelle in the front of the head (called the anterior fontanelle or the soft spot) and one in the back (called the posterior fontanelle).

The posterior fontanelle closes during the first few months of life. The anterior fontanelle closes at around 18–24 months.

A Word From Verywell

Your child’s bones are changing and growing significantly during childhood and adolescence. You can help your kids develop healthy bones by making sure that they are well-nourished and stay active.

When your child begins eating solid foods, ensure that their diet provides sufficient calcium and vitamin D for their age. As they get older, weight-bearing activities, such as walking, running, hiking, and playing sports, and resistance exercises like lifting weights are great ways to build healthy bones.

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Article Sources
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