Bone & Skeleton Development
Bones start as soft cartilage around week 6 and gradually harden (ossify) throughout pregnancy. Some bones, like the skull, remain soft for birth.
Development Timeline
Weeks 6-40 (continues after birth)
Overview
Your baby's skeleton begins as soft, flexible cartilage that gradually hardens into bone through a process called ossification. This process starts around week 6 and continues through childhood and adolescence. The skeleton provides structure, protects organs, and will eventually produce blood cells.
Interestingly, not all bones harden at the same rate. The long bones in the arms and legs begin ossifying first, while some bones - particularly in the skull - remain partially soft to allow the baby's head to compress slightly during birth. These soft spots (fontanelles) gradually close over the first two years of life.
By week 12, your baby has a fully formed skeleton with all bones present, though many are still cartilage. Bone marrow begins producing blood cells around week 11-12, gradually taking over this job from the liver. The skeleton continues strengthening and hardening throughout pregnancy and after birth.
🗓️ Week-by-Week Milestones
Tap any week to open its full pregnancy guide — baby size, symptoms, and what else is developing that week.
Cartilage skeleton begins forming
Arms and legs have cartilage bones, can bend
Tiny joints forming, baby can move limbs
All bones present as cartilage, ossification beginning
Bones hardening rapidly, visible on ultrasound
Bone marrow producing blood cells
Bones strong enough to support body weight in gravity
Bones firm but skull remains flexible for birth
👀 What to Expect
- •Bones visible on ultrasound from second trimester
- •Skeleton looks complete on 20-week anatomy scan
- •Baby's skull bones can overlap during birth (called molding)
- •Newborn head may be cone-shaped from birth, returns to round quickly
- •You can feel soft spots on baby's head after birth
- •Hands and feet proportionally larger at birth
💡 Tips for Parents
- ✓Adequate calcium intake is crucial for bone development
- ✓Vitamin D helps calcium absorption - sunlight and supplements
- ✓Baby's bones are naturally flexible for birth
- ✓Fontanelles (soft spots) on skull are normal and gradually close
- ✓Weight-bearing exercise during pregnancy benefits both your and baby's bones
- ✓Calcium needs increase to 1000-1300mg daily during pregnancy
- ✓Baby draws calcium from your bones if you're not getting enough
🛍️ Supplements & Reads for This Development Stage
Comfort and preparation products other expecting parents find helpful at this stage. These are convenience picks, not medical advice — always follow your provider's guidance.
Prenatal Vitamins (with Folate & DHA)
Daily folate, iron and DHA support that doctors recommend before and throughout pregnancy.
Check Price on Amazon →DHA Omega-3 Supplement (Prenatal)
Doctors recommend DHA throughout pregnancy for baby's brain and eye development — especially the third trimester.
Check Price on Amazon →What to Expect When You're Expecting
The classic week-by-week guide trusted by millions of expecting parents.
Check Price on Amazon →Amazon Associates Disclosure: As an Amazon Associate, we earn from qualifying purchases at no extra cost to you. These recommendations are editorially chosen comfort and preparation products and are not medical advice.
✨ Amazing Facts
Babies are born with about 300 bones that fuse into 206 adult bones
The collarbone is the most commonly broken bone during birth
Babies' bones are more flexible than adults' - less likely to break
The skull bones overlap during birth to fit through birth canal
Bone development continues until about age 25
❓ Frequently Asked Questions
When do a baby's bones start forming?
Your baby's skeleton begins forming around week 6 as soft, flexible cartilage. The long bones of the arms and legs take shape first, and by week 12 a complete skeleton with all bones present is in place — though most are still cartilage at this stage. Ossification, the process of cartilage hardening into true bone, then continues throughout pregnancy and into adulthood.
When do a baby's bones harden?
Bones begin hardening through ossification around week 12 and harden rapidly from about week 16, when the skeleton becomes clearly visible on ultrasound. Hardening continues through the rest of pregnancy and well beyond — bone development is not complete until around age 25. Some bones, especially in the skull, deliberately stay soft and flexible to allow the head to compress during birth.
How many bones are babies born with?
Babies are born with about 300 bones, compared with 206 in an adult. Many of these are separate pieces of cartilage and bone that gradually fuse together during childhood and adolescence — which is why an adult has fewer. This extra flexibility helps babies pass through the birth canal and grow.
How much calcium do I need during pregnancy for my baby's bones?
Pregnant women generally need about 1,000 mg of calcium per day (1,300 mg for teens). Calcium builds your baby's hardening skeleton, and vitamin D helps you absorb it — so dairy, fortified plant milks, leafy greens, and a prenatal vitamin all help. Importantly, if your intake is low, your baby will draw calcium from your own bones, which is why meeting your daily target protects both of you.
What are fontanelles (the soft spots on a baby's head)?
Fontanelles are the soft spots where the skull bones have not yet fused. They exist because the skull bones stay separate and flexible so the head can mold and compress slightly to fit through the birth canal — which is why a newborn's head can look briefly cone-shaped. The soft spots are normal and protected by a tough membrane; they gradually close over roughly the first two years of life.
Does my baby take calcium from my bones during pregnancy?
If you are not getting enough calcium in your diet, your body will release calcium from your own bones to supply your baby's developing skeleton — your baby's needs are prioritized. This is usually temporary and reverses after pregnancy and breastfeeding, but it is the key reason to meet your daily calcium and vitamin D targets, so your baby's bones form well without depleting yours.
Related Development Topics
👶 Planning Ahead?
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