How Your Baby's Skull, Skeleton, And Bones Develop - BabyCenter

Fact: Babies are way smaller than adults. So it stands to reason, then, that they probably have fewer bones. Incorrect! Humans are born with far more bones than they'll end up having in adulthood. Why?

At birth, your newborn's skeleton is still a work in progress. In fact, it won't be fully developed until they're around 20 years old. Babies are born with more than 275 bones, but adults have only 206. That's because many of your newborn's bones will fuse with each other during those important growth years.

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Read on to find out more about how this process works. (You're welcome for this bit of bar trivia, by the way.)

Key Takeaways

• Babies are born with between 275 and 300 bones.
• These bones will fuse together over the years. A grown adult has 206 bones.
• To ensure your baby is getting enough support for their developing bones, make sure you're eating lots of calcium-rich foods during pregnancy.

How baby bones are formed

During the very early days of your pregnancy, the basic outline of each of your baby's bones is established according to instructions from various genes. By the time you're about 7 weeks pregnant, the map of your baby's entire skeleton has been laid out.

Most bones start out as cartilage, a tough but bendable tissue that eventually hardens into bone. (Adults retain some cartilage, mainly in their joints.) But flat bones – like the skull, collarbone, some facial bones, and parts of the pelvis – start out as thin sheaths of tissue called membranes.

All bones develop in the same way in a process called ossification. At about 10 weeks, bone tissue starts to form as cartilage or membrane. Then, calcium and phosphate – minerals stored in your body and replenished by the foods you eat – are added to the tissue to harden it.

Your baby's bone development timeline

Weeks pregnant	Milestone
5 weeks	First pair of somites appear; they will become part of the spine
7 weeks	Bone outlines for entire skeleton established; cartilage is forming
8 weeks	Somites disappear; joints start forming
10 weeks	Bone tissue forms and starts hardening (ossification)
16 weeks	Your baby can move their limbs
16 to 40 weeks	Bone tissue continues to grow; at birth, your baby has more than 275 bones

When does your baby's skull develop?

Your baby's skull isn't one large bone, like an adult's. It starts out as separate bones joined by flexible tissue. You'll be able to feel the soft spots between the bones, called fontanels, on the top and back of your newborn's head.

During birth, the softness of the skull bones and the loose connections between them allow the skull to be gently compressed into a bullet-like shape that more easily fits through the birth canal. This is why many newborns have distinctly pointy heads right after they're born. Within a few days, though, the bones rearrange themselves and the skull goes back to a dome-like shape.

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The fontanel on the back of the head usually closes by the time your baby is around 4 months old; the one on top typically doesn't close until after your baby's first birthday.

"My son still has a soft spot and just turned 18 months," says BabyCenter Community member MaddieMay520. "He was 6 weeks early, so I'm guessing he just needs a little more time."

In the first three years of your child's life, the skull grows rapidly as the brain triples in weight and establishes 1,000 trillion nerve connections. The brain continues to grow in size and weight until your child is about 16, and the skull continues to grow until age 20.

When does your baby's spine develop?

Your baby's spine begins as paired blocks of tissue, known as somites, on each side of the developing spinal cord. These blocks will eventually turn into individual spinal bones (vertebrae), as well as the rib cage, muscles, and skin.

The first pair of somites appears when you're about 5 weeks pregnant, and over the next two weeks, the number increases to 35. During this time, it's possible to see the somites during an ultrasound exam; they look like a string of beads. By 8 weeks, the somites disappear as they develop into different types of tissue.

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Your baby's limb bones

When you're between 5 and 6 weeks pregnant, your baby sprouts paddle-like buds that will lengthen and grow into arms. By 8 weeks, your baby has leg buds that look like flippers. Their arms reach their final proportions by about 14 weeks; the legs take a few weeks longer.

At 16 weeks, your baby starts moving their limbs in a coordinated way. Although you may not feel them until around 18 weeks or later, you can see your baby's acrobatics on ultrasound. As your pregnancy progresses, these movements get so pronounced that at times it can feel like your baby is kickboxing! Some research suggests fetal movements help strengthen babies' bones.

How can you support your baby's bone growth during pregnancy?

It's important to get enough calcium and phosphorus to ensure that your baby's bones grow strong and healthy. Vitamin D is also essential because it helps your body use calcium efficiently. Phosphorus deficiency is very rare, but you'll need to check and make sure you're getting enough calcium and vitamin D.

You need 1,000 milligrams of calcium a day. Dairy foods like yogurt, hard cheese, and even ice cream are good sources of the mineral – and provide lots of vitamin D and phosphorus as well. Nondairy food sources include calcium-fortified juice, almond milk, cereal, and bread, as well as salmon, tofu, and leafy greens like kale and bok choy.

BabyCenter Community member CBabe2 found calcium from a surprising source: "Just found 110 mg of calcium in the bagels I eat… score! Maybe I'm not as calcium-deprived as I thought!"

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If you're not getting 1,000 milligrams of calcium from food and your prenatal supplement, ask your healthcare provider whether you should take calcium as a separate supplement. Calcium chews are an easy way to boost your intake.

"I'm surprised by how many prenatal vitamins don't provide the daily calcium requirement of 1,000 milligrams," says Liz Donner, M.D., a pediatric hospitalist and member of the BabyCenter Medical Advisory Board. "It can be difficult to meet those intake goals through diet alone, especially if you don't consume much dairy. Not only is calcium essential for baby's bone development, it also protects against preeclampsia and preterm birth."

The recommended daily amount of vitamin D during pregnancy is 600 international units (IU). Most prenatal supplements contain 400 IUs, but because vitamin D deficiency is common during pregnancy, some experts recommend more. Inadequate vitamin D can lead to abnormal bone growth, fractures, or rickets in newborns.

But talk with your healthcare provider before taking more vitamin D. You may need to be tested to see if you're deficient, and if you are, your provider can let you know how much to take.