It might be hard to believe when you’re being jabbed from the inside by a pointy elbow or when a little bottom is pushing on your bladder, but your baby’s bones don’t grow in fully developed. They are very much formed by what both of you eat over the months (and years!). 

Here’s a step-by-step look at how that tiny, squishy tadpole becomes a bona fide baby.

How many bones does a baby have? 

Babies are born with around 270 to 300 bones at birth. Over time, some of a baby’s bones fuse (i.e. grow) together, so that by the time your little one is in his 20s, he’ll probably have 206 bones.

Do babies have more bones than adults?

Believe it or not, your newborn has nearly 100 more bones than you do! At birth, babies have around 300 bones, while most adults have a total of 206 bones. 

During pregnancy, the skeletal structure that will one day support your baby’s whole body starts out as cartilage, a firm tissue that’s softer and more flexible than bone. This enables your baby to fit through the birth canal — and allows for growth once he arrives. 

As your little one absorbs more calcium (from you during your pregnancy and then from the foods he eats well beyond), his cartilage gradually ossifies to become hard bone. Around the time he’s about 2 or 3 years old, some of his bones begin to fuse together. The process won’t be fully complete until after he’s an adult to allow his entire body to grow. 

What bone are babies born without?

When babies are born, some of their “bones” are technically cartilage that eventually hardens into bone as they grow. Adults still have some cartilage, but far less than newborns. Most of the cartilage in adults is located at the ends of bones to help them to glide smoothly over each other at the joints. Cartilage also gives a flexible shape and structure to places like the ears and nose.

One example of a bone that babies are born without: the kneecap (or patella). The kneecap starts out as cartilage and starts significantly hardening into bone between the ages of 2 and 6 years old. In most cases, several areas of cartilage in the knee begin to harden at the same time and eventually fuse together to form one solid bone.

Are babies born with both sets of teeth?

Babies are born with one full set of 20 baby (or primary) teeth. Their second set of 32 adult (or permanent) teeth begin to grow after birth, starting with the central incisors (the pairs of upper and lower teeth at the very front of the mouth) at about 3 to 4 months old.

How do bones grow?

Here’s a month-by-month look at how your baby’s bones grow during pregnancy.

Month 1

Soon after conception, the embryo differentiates into three layers of cells. The mesoderm, or middle layer, will develop into your baby’s bones — as well as his heart muscles, kidneys and sex organs. 

The inner layer (called the endoderm) becomes your baby's digestive system, liver and lungs. And the ectoderm, or outer layer, develops into the nervous system, hair, skin and eyes.

Month 2

Big changes are happening to your little embryo in the second month. It’s starting to develop a clavicle and parts of a backbone, for starters, while the neural tube forms — the source for parts of the nervous system as well as the spine and skull.

By about week 6 of pregnancy, your little bean is also sprouting arm buds as the legs follow suit. About the only thing that isn’t growing is the tadpole-like tail. That’s shrinking and will eventually disappear — leaving only the tailbone at the base of the spine.

Month 3

During the final weeks of your first trimester, your baby’s skeleton develops a lot. Those little limb buds grow into recognizable arms and legs with bendable joints and well-defined fingers and toes.

Upper limbs tend to lead the way by a few days with lower ones following — the same way motor skills develop after your baby is born, from the top of the body down (lifting head, then pushing up, then crawling, then walking).

Month 4

Your body is now delivering calcium to your baby via the placenta to help his bones harden, strengthen and lengthen. This calcium transfer continues until birth. He needs to absorb about 30 grams of this important mineral from you over the course of nine months.

Months 5 and 6

These are active months for baby bone-building. Your baby can now wiggle his limbs and you may begin to feel those first flutters of fetal movement sometime around week 18 (give or take).

If you get a peek at your baby during your week 20 ultrasound, you’ll be able to see those bones he’s busy building.

Months 7 and 8

Keep downing those dairy products because the majority of the calcium your baby gets from you is transferred during the third trimester — about 250 milligrams a day! He’s busy transforming cartilage to bone as well as developing muscle and building up a protective layer of fat.

Month 9

At week 36 of pregnancy, mother-baby calcium transfer is peaking with you passing along as much as 350 milligrams of the mineral every day for the remainder of your pregnancy. That’s the case even though your baby’s bones are still softer than an adult’s. 

All of your baby’s bones are designed for delivery, but that’s especially the case for his skull: It’s made up of several separate bony plates that can shift and compress as his head makes its way toward the exit.

Newborn 

As your baby grows after birth, his bones will too. They are living parts of his body, after all! 

One place you’re likely to notice a difference? Your baby’s head. It usually takes two to four months after your baby’s birth for his back “soft spot” (or fontanelle) to close and up to 18 months or more for the front fontanelle to close. 

This allows the skull to expand and keep pace with his rapidly growing brain. Even after the soft spots close, there will still be some room along the sutures (the seams between the bones of the skull).

How to support your baby’s bone growth

During pregnancy, make sure you’re getting about 1,000 milligrams (or 1 gram) of calcium a day to help your baby’s bones grow.

Check your prenatal vitamin to see how much calcium it includes, and make sure to chow down on foods rich in this important mineral, like milk, yogurt, cheese, canned salmon, leafy greens and edamame. You'll also need plenty of vitamin D (found in foods such as canned tuna, eggs, mushrooms and fortified cereal, milk and orange juice) to help your body absorb it. Also ask your doc whether you should take additional calcium supplements.

After you give birth, continue to take your prenatal vitamin for you and your baby if you’re breastfeeding. (Babies get all the calcium they need from breast milk and formula, though they need vitamin D supplements if they're exclusively breastfed). 

And when you switch your little one to table food, keep the calcium coming well through high school and beyond for lifelong healthy bones.