A newborn enters the world with roughly 270 to 300 separate bones and cartilage elements, yet an adult skeleton contains only 206. The difference comes from bone fusion, a process that begins before birth and continues, in some parts of the body, well into your twenties. Fusion does not happen all at once or in a single region. It follows a loose schedule that starts at the skull and jaw within months of birth, sweeps through the spine and pelvis during childhood and adolescence, and finishes at surprisingly late outposts like the collarbone and the uppermost sacral vertebrae.
The Skull and Face
The earliest fusions most people have heard of involve the skull. A baby’s skull is not one solid shell but several flat bones separated by fibrous gaps called sutures and wider soft spots called fontanelles. The anterior fontanelle, the large diamond-shaped soft spot on the top of a baby’s head, closes progressively from birth, with the steepest rate of shrinkage occurring between about three and nine months of age.1PubMed Central. Temporal mapping of the closure of the anterior fontanelle and contiguous sutures using computed tomography, in silico models of modern infants In most children the fontanelle is fully closed by around 17 to 18 months.2PubMed Central. Difference in anterior fontanelle closure between non-syndromic craniosynostosis and normal controls: a retrospective cross-sectional study Closure here does not mean the skull sutures themselves have fused into solid bone; the sutures remain partially open for years and some stay slightly flexible into adulthood. Full bony obliteration of cranial sutures is a much slower, decades-long process.
Down at the chin, a less well-known fusion happens even earlier. The mandible (lower jaw) starts life as two separate halves joined by a midline suture called the mandibular symphysis. Evidence from archaeological infant remains places fusion of this suture at around seven to eight months of age, with a range of about six to nine months.3PubMed. Mandibular symphysis (medial suture) closure in modern Homo sapiens: preliminary evidence from archaeological populations By the time a child is walking, the jawbone is already one continuous piece.
Deeper inside the skull, the spheno-occipital synchondrosis links the sphenoid bone (behind the eyes) to the occipital bone (the back of the skull base). This cartilaginous joint is the last growth center in the skull base, and its closure tracks with puberty. On average, the synchondrosis is still open at around age 11 in boys and close to 10 in girls, partially fused by the early teens, and completely fused by about 16 in boys and 15 in girls.4PLOS ONE. Timing and rate of spheno-occipital synchondrosis closure and its relationship to puberty Girls tend to run one to three years ahead of boys at each stage.5PubMed Central. Time and pattern of the fusion of the spheno-occipital synchondrosis in patients with skeletal Class I and Class III malocclusion That said, a comprehensive review of sixty years of research found the timing is highly variable across individuals: an open synchondrosis has been documented in people up to their mid-twenties, and forensic scientists are cautioned against declaring it always closes in adolescence.6PubMed. When does the spheno-occipital synchondrosis close?
The Spine
Each vertebra in a child’s spine is actually made of multiple pieces. The vertebral body and the two halves of the neural arch (the ring that protects the spinal cord) are connected by cartilaginous joints called neurocentral synchondroses. These close in a top-down wave. In the cervical spine (the neck), the synchondroses are essentially closed by around age five. The lumbar spine (lower back) follows, finishing by roughly age ten or eleven. The thoracic spine is the slowest, remaining partly open through at least age seventeen.7PubMed. Growth Patterns of the Neurocentral Synchondrosis (NCS) in Immature Cadavaric Vertebra This matters clinically because any procedure that disrupts an open neurocentral synchondrosis in a growing child risks asymmetric vertebral growth and spinal deformity.
The vertebrae also have ring-shaped secondary ossification centers at their top and bottom surfaces (ring apophyses), which fuse during late adolescence. These are separate from the neurocentral synchondroses and contribute to the final height of each vertebral body.
The Sacrum and Pelvis
The sacrum is a single wedge-shaped bone at the base of the adult spine, but it begins as five separate vertebrae. Fusion starts at the lower segments and works upward. The joint between the first and second sacral vertebrae (S1/S2) is the slowest to close: in one imaging study, 80 percent of men had achieved bony fusion at that level by about age 29, while women reached the same milestone by about age 25.8PubMed Central. Age-specific normative values of sacral development and fusion in children and adolescents: a cross-sectional study utilizing multiplanar reconstruction computed tomography imaging Women generally fuse the sacral segments earlier than men, though both sexes eventually reach the same endpoint.9PubMed. Sacral fusion as an aid in age estimation The sacrum’s late-finishing timeline surprises many people; it means a portion of your spine is technically still solidifying through your twenties.
Each hip bone tells a similar story. What adults think of as one solid hip bone is actually three separate bones in a child: the ilium (the broad wing you feel at your waist), the ischium (the bone you sit on), and the pubis (the front). All three meet at the acetabulum, the cup-shaped socket for the thigh bone.10PubMed. Ontogeny of the Human Pelvis Fusion at the acetabulum typically completes during the mid-teenage years, though secondary ossification centers on the iliac crest and other pelvic margins can remain unfused until the early twenties.
The Sternum
The breastbone also begins life in pieces. The body of the sternum develops from four separate segments called sternebrae, and these fuse from the bottom upward over a long stretch of time. The lowest two sternebrae begin merging in early childhood, and complete fusion between them is seen in all specimens by the mid-teens. The upper segments take much longer. Complete fusion of the top two sternebrae was first observed at age 16 in males and 18 in females, and full fusion of every segment was not universal in males until after age 60 in one study of skeletal remains from northwest India.11PubMed. Time of fusion of the human sternebrae with one another in northwest India This is one of the body’s most drawn-out fusion events, and it explains why radiologists sometimes see unfused sternal segments in healthy adults and mistake them for fractures.
The Collarbone and Long Bones
The medial end of the clavicle, where it meets the sternum, is often cited as the last growth plate in the human body to close. Its epiphyseal ossification center does not even appear until around age 11, partial fusion occurs between 16 and 23, and complete fusion was first observed at 18 in males and 19 in females in one CT study.12PubMed Central. CT evaluation of medial clavicular epiphysis as a method of bone age determination in adolescents and young adults Because it finishes so late, the medial clavicular epiphysis is used in forensic science to help estimate whether an individual has reached legal adulthood.
The long bones of the arms and legs, such as the femur, tibia, humerus, and radius, each have growth plates (physes) near their ends. These cartilage zones are what allow your limbs to lengthen throughout childhood and adolescence. Growth plate fusion here follows a roughly predictable sequence: the elbow fuses before the wrist, and the ankle before the knee. Most long-bone growth plates close between ages 14 and 18, though some finish a few years later. The process is driven by the gradual replacement of cartilage with bone, a mechanism called endochondral ossification.13The International Journal of Biochemistry & Cell Biology. Endochondral ossification: How cartilage is converted into bone in the developing skeleton
Why Estrogen Controls the Clock
A key question behind all of this fusion is: what tells a growth plate to stop growing and seal shut? The dominant signal is estrogen. Both boys and girls produce estrogen (boys convert testosterone into it), and rising estrogen levels during puberty are the trigger for growth plate closure throughout the skeleton.14PubMed Central. Pubertal growth and epiphyseal fusion Estrogen does not simply flip a switch. Instead, it accelerates the natural aging of the cartilage cells in the growth plate, pushing them through their limited number of divisions more quickly. Once those cells have exhausted their ability to multiply, the cartilage thins and bone fills in the gap.15PubMed. Effects of estrogen on growth plate senescence and epiphyseal fusion
This explains several things at once. It explains why girls, who enter puberty earlier and experience a steeper rise in estrogen, tend to fuse their growth plates one to three years before boys. It explains why children who go through precocious (early) puberty can end up shorter as adults: their growth plates close before they have had enough time to grow. And it explains why individuals with rare conditions that block estrogen signaling can have growth plates that remain open into their thirties, resulting in unusually tall stature.
The Hyoid Bone, a Lifelong Wild Card
Not every bone in the body follows a tidy fusion schedule. The hyoid, a small horseshoe-shaped bone in the throat that supports the tongue and larynx, consists of a central body and two pairs of projections called the greater and lesser horns. Fusion of the greater horns to the body is famously unreliable. In one study, no fusion at all was observed before age 25 in either sex.16PubMed. Time of fusion of greater cornu with body of hyoid bone in Northwest Indians After that age, the pattern becomes erratic. Some people show full bilateral fusion by their thirties; others retain unfused horns past age 60. A separate study found that bilateral non-fusion was present in about 63 percent of young adults, dropping to roughly 16 percent in older adults, but even among older adults bilateral fusion was not the universal outcome.17PubMed Central. Hyoid bone fusion and bone density across the lifespan: prediction of age and sex
This variability has real-world consequences in forensic pathology. A fractured hyoid bone is sometimes cited as evidence of strangulation, but distinguishing a fracture from a naturally unfused joint requires knowing the person’s baseline fusion status, which is inherently unpredictable. Forensic scientists have concluded that using hyoid fusion to estimate age is unreliable.16PubMed. Time of fusion of greater cornu with body of hyoid bone in Northwest Indians
When Fusion Happens Too Early or Not at All
The orderly schedule of bone fusion can go wrong in both directions. The best-known example of premature fusion is craniosynostosis, a condition in which one or more skull sutures fuse before the brain has finished growing. This forces the skull into an abnormal shape: growth is restricted at the fused suture and compensatory bulging occurs elsewhere.18PubMed Central. Craniosynostosis In the study comparing children with craniosynostosis to normal controls, the affected group showed much higher rates of anterior fontanelle closure before age two, with the most dramatic difference at 10 to 12 months (50 percent closed, versus about 7 percent in the control group).2PubMed Central. Difference in anterior fontanelle closure between non-syndromic craniosynostosis and normal controls: a retrospective cross-sectional study Researchers have identified many gene mutations that cause syndromic craniosynostosis, though the genetic basis for the majority of non-syndromic cases remains unknown.19PubMed Central. Understanding craniosynostosis as a growth disorder
On the other end of the spectrum, some bones that should separate during fetal development fail to do so. Tarsal coalition is a condition in which two or more of the small bones in the foot remain abnormally connected, caused by a failure of the tissue between them to properly segment during embryonic growth.20PubMed Central. Surgical treatment of tarsal coalitions in children and adolescents The connections can be bony, cartilaginous, or fibrous. Calcaneonavicular and talocalcaneal coalitions are the most common types.21PubMed. Congenital tarsal coalition: multimodality evaluation with emphasis on CT and MR imaging Children with tarsal coalition often have no symptoms until adolescence, when the cartilaginous bridge begins to ossify and the foot stiffens. At that point the classic presentation is a rigid flat foot with pain during activity. Mild cases can be managed with orthotics and physical therapy; severe or symptomatic cases may require surgical resection of the bridge or, in some instances, joint fusion on the surgeon’s terms rather than the body’s.
Bone Fusion and Forensic Age Estimation
Because different bones fuse on a roughly predictable timetable, skeletal fusion has been used for centuries to estimate age, especially in archaeological, forensic, and immigration contexts. The general approach involves assessing where a person falls along the appearance and fusion of various ossification centers.22Academic Press. Skeletal age estimation in juveniles and subadults The medial clavicular epiphysis, the spheno-occipital synchondrosis, and sacral vertebral fusion are all commonly examined because their late closure overlaps with the legal threshold of adulthood in many jurisdictions.
The reliability of these methods varies. Sacral fusion and clavicular epiphysis fusion are reasonably consistent across populations, though women tend to fuse earlier. The spheno-occipital synchondrosis is less reliable at the individual level because of its wide range of closure times. And as already noted, the hyoid bone is essentially useless for aging. Population differences in nutrition, hormonal environment, and genetic background all add noise to the schedule, so forensic age estimates are typically given as a range rather than a single year.
A Fusion Unique to Humans and African Apes
One fusion that most people never think about is the merging of two small wrist bones, the scaphoid and the centrale. In most primates these remain separate throughout life, but in humans and African apes (chimpanzees and gorillas) they fuse early, either before birth or in the first months of life. This pattern is not seen in Asian apes like orangutans. Research on the timing and frequency of scaphoid-centrale fusion across primate species suggests the trait is an evolutionary shared feature of the African ape-human lineage, not a random occurrence, and it appears to have shifted earlier in development over evolutionary time.23PubMed. Frequency and timing of scaphoid-centrale fusion in hominoids The functional significance is debated, but a fused scaphoid-centrale may stiffen the wrist in a way that helps stabilize it during knuckle-walking or forceful gripping. In humans, you would never know it happened; by the time your wrist bones show up on an X-ray, they look like they were always a single piece.