A baby’s bones start hardening in the womb, around the sixth to seventh week of pregnancy, but the full process isn’t complete until the mid-twenties. Newborns arrive with 275 to 300 bones, many of them soft cartilage or partially mineralized tissue. Over the next two decades, those bones gradually harden, fuse together, and consolidate into the 206 bones of an adult skeleton.
Bone Formation Starts Before Birth
Ossification, the process of turning soft tissue into hard bone, begins between weeks six and seven of embryonic development. The flat bones of the skull and the collarbones form first through a direct conversion of soft tissue into bone. The long bones of the arms and legs take a different route: they start as cartilage models that are gradually replaced by mineralized bone tissue. By the time a baby is born, primary hardening centers are active in most bones, but large portions of the skeleton remain cartilage.
This mix of cartilage and bone is part of the design. A skeleton made entirely of rigid bone would make birth nearly impossible and would limit how quickly a baby can grow in the first years of life. The soft, flexible parts act as built-in expansion joints.
The Soft Spots on a Baby’s Head
The fontanelles, or soft spots, are the most visible example of bones that haven’t yet fused. A newborn has several of these gaps between skull bones, and they close on a predictable schedule:
- Posterior fontanelle (back of the head): closes within about six to eight weeks after birth.
- Sphenoid fontanelle (side of the head, near the temple): closes around six months.
- Mastoid fontanelle (behind the ear): closes between six and eighteen months.
- Anterior fontanelle (top of the head): the largest soft spot, closing between 13 and 24 months.
The anterior fontanelle is the one parents notice most. It’s normal for it to remain open well into a toddler’s second year. If you feel a hard ridge along any of the skull’s seams in a young infant, or if the soft spots seem absent entirely, that can signal craniosynostosis, a condition where skull bones fuse too early. It occurs in roughly 1 in 2,500 babies and can affect head shape and, in some cases, brain growth. The earliest sign is usually an abnormally shaped skull or a head that isn’t growing as expected.
Bones That Stay Soft for Years
Some bones remain cartilage far longer than most parents expect. Kneecaps are a striking example. Babies and toddlers don’t have bony kneecaps at all. The patella begins hardening between ages three and five, starting as several small bony spots that merge together. This is why a toddler’s tumble onto their knees, while it may cause tears, rarely causes the kind of fracture an adult might get.
Much of a child’s wrist and hand skeleton also remains cartilaginous through early childhood. Pediatricians sometimes use a hand X-ray to assess “bone age,” because the pattern of which small bones have hardened reveals a lot about a child’s overall skeletal development.
Growth Plates and the Final Stage
The last parts of the skeleton to fully harden are the growth plates, thin layers of cartilage near the ends of long bones. These plates are what allow bones to lengthen throughout childhood and adolescence. As long as a growth plate remains open, the bone can still grow.
Growth plates close in a roughly predictable sequence, and girls tend to finish one to two years ahead of boys. At the knee, for instance, complete fusion of the thighbone’s growth plate occurs by age 20 to 21 in females and 21 to 22 in males. The shinbone and the smaller bone beside it follow roughly the same timeline. Other growth plates around the body close at different ages, but the general pattern holds: most fusion happens during and just after puberty, with the final plates solidifying by the mid-twenties.
This is why the skeleton of a 16-year-old, though it may look fully formed on the outside, still contains active growth zones that are vulnerable to certain types of injuries. Fractures through a growth plate need careful attention because they can disrupt future bone lengthening if they heal improperly.
What Drives Bone Hardening
Several hormones control how quickly bones mature. Growth hormone and a related compound called IGF-1 are the primary drivers during childhood, stimulating the cartilage-to-bone conversion process. Thyroid hormone also plays a key role. Children with an underactive thyroid often show delayed bone development, while an overactive thyroid can accelerate it.
During puberty, sex hormones (estrogen in girls, testosterone in boys) ramp up bone hardening significantly. Estrogen, in particular, is responsible for eventually sealing the growth plates shut. This is why children who enter puberty early tend to stop growing sooner, and why those with delayed puberty often have a longer window of bone growth.
How Movement and Nutrition Help
Physical activity plays a direct role in building stronger, denser bones. When a baby progresses from lying down to sitting, crawling, pulling up, and eventually walking, each new milestone increases the mechanical load on the skeleton. That loading signals bones to deposit more mineral and grow thicker. Research in pediatric rehabilitation has shown that even passive weight-bearing exercise increases bone mineral density in children who can’t walk on their own, confirming that the skeleton responds to physical stress at every stage of development.
Nutrition is equally important. Calcium provides the raw building material, and vitamin D is essential for absorbing it. Babies under 12 months need 400 IU of vitamin D daily. Breastfed infants and those receiving a mix of breast milk and formula typically need a supplement to reach that level, since breast milk alone doesn’t supply enough. Formula-fed babies usually get adequate vitamin D from their formula, provided they drink enough each day. Without sufficient vitamin D, bones can remain soft and poorly mineralized, a condition known as rickets.
The Full Timeline at a Glance
- 6 to 7 weeks of pregnancy: Ossification begins in the embryo.
- Birth: Baby has 275 to 300 partially hardened bones.
- 6 weeks to 24 months: Skull fontanelles gradually close.
- 3 to 5 years: Kneecaps begin hardening from cartilage to bone.
- Throughout childhood: Smaller bones fuse into larger ones; cartilage is steadily replaced.
- Puberty through early twenties: Growth plates close; the last bones finish hardening.
- Around age 25: The skeleton reaches full maturity.
The process is remarkably long. A baby’s skeleton at birth is really just a rough draft, with two full decades of remodeling ahead before it reaches its final, fully hardened form.