Your Baby's Skeleton: How Many Bones Does A Baby Have?

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 that your baby's bones are still soft, flexible, and a long way from their final form. In fact, they're a work in progress — and will be for years after birth.

Here's a look at how your little one's skeleton develops before birth and what you can do to help build strong bones that last a lifetime.

Key Takeaways:

• Your baby starts out with more bones than you do. At birth, babies have nearly 300 bones — about 100 more than adults! Over time, some of those tiny bones fuse together, leaving most grown-ups with 206.
• Bone building begins early (and continues well after birth). From the moment that mesoderm layer forms in the first month, your baby's skeleton is on its way. Calcium from your diet plays a starring role throughout pregnancy — and even after delivery, when bones keep growing, fusing, and hardening all the way into young adulthood.
• Nutrition matters for strong bones — now and later. During pregnancy, you'll need about 1,000 milligrams of calcium (plus vitamin D to help absorb it) each day to support your baby's developing skeleton. Keep up the calcium-rich foods through breastfeeding and beyond to set your little one up for lifelong bone health.

How many bones does a baby have?

Your baby actually starts life with more bones than you! Babies are born with around 270 to 300 bones, while adults typically have 205. That's because many of a baby's smaller bones fuse together as they grow into childhood, adolescence, and adulthood.

Some of a newborn's "bones" are actually cartilage that eventually hardens into bone as they grow. (Cartilage is firm tissue that's softer and more flexible than bone, which comes in handy when your baby journeys through the birth canal.)

As your baby absorbs more calcium (from you during pregnancy and then from the foods he eats as a child), his cartilage gradually hardens into bone. And around the time he's 2 or 3 years old, some of his bones begin to fuse together. The process isn't fully complete until he's an adult, giving his body plenty of time to grow.

The kneecap (patella) is a good example. In most cases, several areas of cartilage in the knee begin to harden at the same time and eventually fuse together — between the ages of 2 and 6 years — to form one solid bone.

Not every baby is born with the exact same number of bones, either. "Some babies — often those of African and Caribbean descent — may be born without a nasal bone, for example," notes Jennifer L Wu, M.D., an OB/GYN in New York City and a member of the What to Expect Medical Review Board.

How do my baby's bones grow in utero?

Here's a month-by-month peek at how your baby's bones grow during pregnancy:

Month 1

Soon after conception, your baby's body begins forming from three layers of cells. The middle layer, called the mesoderm, is where bones start — along with the heart, kidneys, and sex organs.

Those other layers are specialized, too. The inner layer (called the endoderm) becomes your baby's digestive system, liver, and lungs. And the outer layer (ectoderm), develops into the nervous system (brain, spinal cord, and nerves), hair, skin, and eyes.

Month 2

Big changes are happening! The neural tube is forming, along with your baby's first bones: the clavicle and backbone.

By about week 6 of pregnancy, your little bean is developing tiny buds that will become arms and legs.

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.

The first trimester is an important time for your baby's development, and it's a time when he's most at risk for damage from things like heavy alcohol consumption, illegal drug use, certain medicines, and infections such as rubella, according to the National Institutes of Health (NIH).

Month 3

During the final weeks of your first trimester, your baby's skeleton looks much more familiar. Those limb buds lengthen into recognizable arms and legs, complete with bendable joints and well-defined fingers and toes.

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

Month 4

Your body is now delivering calcium to your baby via the placenta. For healthy bone development, your baby needs to absorb about 30 grams of this important mineral from you between now and birth.

It's during this month that your baby's bones lengthen and harden — especially the long bones, according to the American College of Obstetricians and Gynecologists (ACOG). Limb movements are becoming more coordinated too.

Months 5 and 6

Your baby is wiggling! You may begin to feel those first flutters of fetal movement (called "quickening") around week 18.

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, calcium transfer is peaking, with you passing along as much as 350 milligrams of the mineral to your baby every day for the remainder of your pregnancy.

Even though your baby's bones are still softer than an adult's, they're ready for delivery. The skull, in particular, is designed for birth: Its separate plates can gently shift during labor to help your baby make his grand entrance.

Newborn

Bone development doesn't stop once your baby is born.

In fact, you'll notice soft spots (fontanelles) on your newborn's head. These allow the skull to expand as your baby's brain grows rapidly in the first year of life. The back soft spot usually closes within a few months, while the front one may stay open for up to 18 months or longer.

Throughout childhood and adolescence, bones will continue to grow, fuse, and harden until your child reaches adulthood.

Are babies born with both sets of teeth?

Babies are born with one full set of 20 baby (or primary) teeth in their gums. 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.

Get week-by-week updates on how your baby is growing and developing in there in the What to Expect app.

How to support your baby's bone growth

To support your baby's bone growth, focus on calcium:

• Eat calcium-rich foods: Aim for about 1,000 milligrams (or 1 gram) a day during pregnancy. Great sources include milk, yogurt, cheese, leafy greens, canned salmon, and edamame.
• Don't forget vitamin D: It helps your body absorb calcium. You'll find it in foods like fortified milk and orange juice, canned tuna, eggs, mushrooms — and of course, sunlight.
• Take your prenatal vitamin: While you still need food sources, many prenatal vitamins contain some calcium to boost your intake. Ask your provider if extra supplements are a good idea.
• Keep it going postpartum: Continue your prenatal vitamin if you're breastfeeding. And once your baby starts solids, keep calcium-rich foods on the menu well into the teen years for lifelong strong bones.

So while those little kicks, stretches, and squirms may feel mighty powerful right now, those little bones are still soft — and you're helping make them strong for all the running, climbing, and adventures that lie ahead.