Which Bones Are Babies Born Without?

Babies are born with roughly 270 to 300 separate skeletal elements, far more than the 206 bones in a typical adult skeleton. The difference comes not from bones appearing later in life but from many infant bones starting as cartilage that gradually hardens, and from separate pieces that eventually fuse together. The most commonly cited “missing” bone is the kneecap, but wrist bones, certain ankle bones, and parts of the spine and pelvis also begin as cartilage templates or unfused segments that only become recognizable bone over months or years.

The Kneecap Is the Classic Example

Ask anyone which bone babies lack, and the answer is almost always the kneecap, or patella. This is largely correct, with a caveat. Newborns do have a kneecap-shaped structure in the right place, but it is made entirely of cartilage rather than bone. Because cartilage does not show up on a standard X-ray, the kneecap appears to be absent when you image an infant’s knee. Research on the developmental timeline in mammals shows that the patella begins as a cartilaginous structure during fetal life, with ossification (the process of cartilage being replaced by true bone tissue) starting only after birth and continuing for several years.¹ In humans, the ossification center in the kneecap typically appears between ages three and five, and the bone does not fully harden until late childhood or early adolescence.

So when people say babies are born without kneecaps, what they really mean is that babies are born without bony kneecaps. The cartilage version is there from early fetal development, doing essentially the same structural job, just in a softer material. This distinction matters because it means the knee joint is not unprotected in infancy. The cartilaginous patella still guides the tendon over the joint and helps with leg extension, even though it would look like empty space on an X-ray.

Wrist and Ankle Bones Take Years to Appear

The wrist is another area where babies have almost no visible bone. Adults have eight small carpal bones arranged in two rows, and none of them are ossified at birth. They show up gradually over the first decade of life. A radiological study of children found that the first wrist bones to develop visible ossification centers, the capitate and hamate, appear during the first year. Others follow at roughly yearly intervals, and the last one, the pisiform, does not show up until around age ten.² That means a five-year-old’s wrist X-ray will show only a handful of bony dots surrounded by what looks like empty space, even though the cartilage templates for all eight bones are already in position.

A similar process happens in the ankle and foot. The tarsal bones ossify on a slightly different schedule, with some beginning during fetal life, but several do not complete the process until well into childhood. Pediatricians and radiologists use this predictable sequence as a tool: a wrist or hand X-ray can help estimate a child’s skeletal maturity, which is useful when growth disorders or hormonal conditions are suspected. The technical term for this is a “bone age” assessment, and it works precisely because the timing of ossification is consistent enough across healthy children to serve as a biological clock.

Fontanelles Are Gaps, Not Missing Bones

The soft spots on a baby’s head, called fontanelles, are another feature people often describe as missing bone. Fontanelles are areas where the flat bones of the skull have not yet grown together, leaving gaps covered by tough fibrous membrane. They exist for two practical reasons: they allow the skull to compress slightly during birth so the head can pass through the birth canal, and they give the rapidly growing brain room to expand without being squeezed by rigid bone.³

The largest soft spot, the anterior fontanelle at the top of the head, typically closes between twelve and eighteen months of age. The posterior fontanelle at the back of the skull usually closes within the first few months. These are not cases of bone being absent; the skull bones are present, they just have not expanded enough to meet their neighbors. As the bones grow outward from their centers, they eventually overlap and fuse at joints called sutures. If fontanelles close too early (a condition called craniosynostosis), the skull cannot accommodate brain growth properly, which can require surgical intervention. If they close too late, it can signal nutritional deficiencies, hormonal issues, or genetic conditions. So the timing of fontanelle closure is one of the things pediatricians check during routine well-baby visits.

How Cartilage Turns Into Bone

Most of the skeleton develops through a process where cartilage serves as a temporary scaffold that gets progressively replaced by true bone tissue. Cartilage cells multiply, enlarge, and eventually die, leaving behind a matrix that gets invaded by blood vessels and bone-building cells called osteoblasts, which then lay down actual bone.⁴ This is how long bones like the femur and humerus grow, and it is why children have growth plates, which are strips of active cartilage near the ends of bones that keep producing new tissue until adolescence.

Not every bone follows this path. The flat bones of the skull and the collarbones develop through a different route, forming directly from sheets of connective tissue without a cartilage intermediate.⁵ This is why the clavicle is one of the first bones in the body to begin ossifying, starting as early as the fifth or sixth week of fetal life, while the kneecap does not start hardening until years after birth. The two processes run on completely different timelines, which is part of why the question “which bones are babies born without” has such a complicated answer. It depends on whether you count cartilage precursors as “having” the bone or not.

Sesamoid Bones and Other Late Arrivals

Beyond the kneecap, the body contains a number of small bones called sesamoid bones that are embedded within tendons or joint capsules. The most familiar ones, aside from the patella, are the two tiny sesamoids under the big toe joint. These develop under the influence of mechanical forces and local signaling during growth.⁶ Research has found that different types of sesamoid bones share a common cellular origin but follow divergent developmental programs.⁷

None of the smaller sesamoid bones are ossified at birth. Their development is tied to the mechanical demands placed on the skeleton as a child begins to walk, run, and bear weight. Some people develop extra sesamoid bones that others never form, and the total number of sesamoid bones in the body varies from person to person. This is one reason why the commonly cited figure of 206 adult bones is an average rather than a universal constant. Some adults have 205 and some have 210, depending on how many sesamoid bones developed and whether certain fusions happened the typical way.

The Hyoid and Other Bones That Fuse Over Time

The hyoid is a horseshoe-shaped bone in the throat that supports the tongue and plays a role in swallowing and speech. In infants and children, it exists as three separate pieces that have not yet fused. A study examining hyoid bones across the lifespan found no fusion at all before age twenty, with the bone gradually fusing over the following decades.⁸ This matters in forensic medicine, where the degree of hyoid fusion can help estimate a person’s age.

The hyoid is an extreme example of a broader pattern. The sacrum, which forms the back wall of the pelvis, starts as five separate vertebrae that fuse during adolescence and early adulthood. The hip bone begins as three distinct bones (the ilium, ischium, and pubis) that do not fully fuse until the mid-teens. The sternum, or breastbone, starts as several separate segments. Even the vertebrae themselves each begin as three separate ossification centers that join together during childhood. All of these unfused elements contribute to the higher bone count at birth. Babies do not gain bones as they grow; they lose them, as separate pieces merge into single structures.

Why So Much Cartilage at Birth

A skeleton made partly of cartilage is not a design flaw. It serves the newborn in several ways. Cartilage is more flexible than bone, which helps during the tight squeeze of vaginal delivery. It is also lighter relative to its volume, which matters when you consider that a newborn’s head is proportionally enormous compared to the rest of its body. A fully ossified skull at birth would make delivery far more difficult and dangerous for both mother and baby.

Cartilage also accommodates the dramatic growth that happens in the first few years of life. Bone is rigid once laid down and can only grow by adding material at its surfaces or at specialized growth zones. Cartilage is more metabolically active and can expand from within, which allows the skeleton to keep up with the rapid increase in body size. By the time growth slows in adolescence, most of the skeleton has converted to bone, and the remaining growth plates close, signaling the end of height gain.

What Doctors See When Imaging a Baby’s Skeleton

The fact that so much of the infant skeleton is cartilaginous creates a practical challenge for medical imaging. Standard X-rays only show mineralized bone, so large portions of a baby’s skeleton are invisible on a radiograph. This can make it difficult to diagnose fractures that involve cartilaginous areas, which is where ultrasound becomes valuable. Ultrasound can visualize the non-ossified cartilage structures that X-rays miss, and it does not expose the child to radiation.⁹

This is especially relevant when an infant or young child has a suspected injury near a joint. A fracture that extends into the unossified cartilage at the end of a bone would be invisible on an X-ray but can be picked up by ultrasound, which can show the alignment of those soft structures and detect whether the growth plate has been disrupted.¹⁰ MRI is another option for detailed imaging of cartilage, but it often requires sedation in very young children, making ultrasound the preferred first-line tool for many pediatric musculoskeletal questions.

When Bones Are Truly Absent

Everything discussed so far involves bones that are present in cartilage form and simply have not yet ossified. But there are genuine medical conditions where bones fail to develop at all. Patellar aplasia, the complete absence of the kneecap, occurs in several genetic syndromes. A review of these conditions found multiple genetic diseases with identified molecular bases that include abnormal, underdeveloped, or completely absent kneecaps as part of the clinical picture.¹¹ Nail-patella syndrome is probably the best known of these, where affected individuals are born without kneecaps and also have abnormalities in their fingernails and other skeletal structures.

Other conditions involve absent or extra bones in the hands, feet, or spine. Some people are born with extra ribs (cervical ribs attached to the neck vertebrae), while others are missing a rib pair. Absent or fused vertebrae occur in conditions like Klippel-Feil syndrome. These are relatively rare, but they illustrate that the normal pattern of skeletal development can go off-script in ways that go beyond the typical cartilage-to-bone conversion timeline.

Nutrition and Hormones in Early Bone Development

The conversion of cartilage to bone is not purely on autopilot. It depends on adequate nutrition, especially calcium, phosphorus, and vitamin D, along with normal hormone levels. Thyroid hormones are particularly important for skeletal maturation in children. Research on children with congenital hypothyroidism, a condition where the thyroid gland does not produce enough hormone from birth, has examined whether long-term thyroid hormone replacement disrupts normal bone turnover. One study found that with effective vitamin D and thyroid hormone replacement, the condition did not appear to harm bone remodeling.¹² The takeaway is that when hormonal or nutritional deficiencies are caught early and treated, the skeleton can develop on a relatively normal schedule.

Rickets, caused by severe vitamin D deficiency, is the most dramatic example of what happens when bone development is nutritionally compromised. The cartilage at growth plates fails to mineralize properly, leading to soft, deformed bones. While rickets has become much less common in developed countries thanks to vitamin D supplementation and fortified foods, it has not disappeared entirely and remains a concern in populations with limited sun exposure or restrictive diets.

Precocial Animals and the Evolutionary Trade-Off

Humans are born with unusually immature skeletons compared to many other mammals. A foal can stand and run within hours of birth; a human baby cannot even hold up its own head. This reflects a fundamental evolutionary trade-off. Humans have exceptionally large brains relative to body size, and the pelvis can only accommodate so much head circumference. The solution is to be born relatively early in development, with a skull that can still deform and a skeleton that is far from finished. Research on precocial mammals, those born in a more mature state, has shown that their skeletal systems are functionally robust at birth in ways that infant human bones are not.¹³

This evolutionary context helps explain why so much of the human infant skeleton remains cartilaginous. It is not that the bones are delayed; it is that the entire developmental timeline is shifted to allow for early birth and prolonged postnatal brain growth. The trade-off is a long period of physical vulnerability and dependence, compensated by a much longer window for brain development outside the womb.

What Neanderthal Babies Tell Us

Fossil evidence from Neanderthal infants offers a fascinating comparison point. Analysis of bone microstructure in a late-term Neanderthal fetus found skeletal tissue that was remarkably similar to what you would see in a modern human fetus of the same gestational age, with some regions showing slightly more advanced bone formation than expected for the developmental stage.¹⁴ Meanwhile, study of a Neanderthal infant skeleton found signs of unusually rapid body growth in early life, suggesting that Neanderthals followed a developmental strategy distinct from modern humans, with faster somatic growth in infancy.¹⁵

These findings suggest that while the basic blueprint of fetal bone development is shared across closely related hominin species, the pace and trajectory can differ. Neanderthal infants may have been born with slightly more mature skeletons than modern human newborns, consistent with their stockier, more robust adult builds. The research is based on extremely rare and fragmentary fossils, so conclusions remain tentative, but it adds a layer of perspective: the particular degree of skeletal immaturity we see in modern human babies is not the only way to build a hominin. It is one solution among several that evolution has tried.