Most people have five lumbar vertebrae, the chunky bones that form the inward curve of your lower back between the ribcage and the pelvis. This is the number you’ll find in every anatomy textbook, and it holds true for the vast majority of the population. But “most” is doing real work in that sentence, because a surprisingly large fraction of people quietly carry four or six lumbar vertebrae without ever knowing it, and that mismatch can create real problems when surgeons, radiologists, and pain specialists try to pinpoint exactly which spinal segment is causing trouble.
What the Lumbar Vertebrae Do
Your spine is divided into regions from top to bottom: seven cervical vertebrae in the neck, twelve thoracic vertebrae in the mid-back (each attached to a rib), five lumbar vertebrae in the lower back, and then the fused bones of the sacrum and coccyx at the base. The lumbar segment bears the most weight. Every time you stand, bend, lift, or twist, the lumbar vertebrae and the discs between them absorb and distribute the load from your entire upper body down into your pelvis and legs.
Because they handle so much force, the lumbar vertebrae are the largest and thickest in the entire spine. They lack the rib attachments of thoracic vertebrae, which gives the lower back far more range of motion but also makes it more vulnerable to injury. The discs between them are also the thickest in the body, acting as shock absorbers. When people talk about a “slipped disc” or “bulging disc,” they’re almost always referring to a lumbar disc, and the specific vertebra involved matters a great deal for treatment.
When the Count Is Not Five
The textbook count of five lumbar vertebrae is the modal number, not a universal one. Variation in the total number of mobile vertebrae in the spine is more common than most people assume. One imaging study of 418 patients who received whole-spine MRI found that about 7.7% had a non-standard number of mobile vertebrae, and these numerical variants were roughly two and a half times more common than the transitional vertebrae that get most of the clinical attention.1PubMed Central. Incidence of numerical variants and transitional lumbosacral vertebrae on whole-spine MRI Another study calculated the prevalence of four lumbar vertebrae at about 1.8% and six lumbar vertebrae at about 0.8%.2PubMed Central. Pelvic Incidence in Spines With 4 and 6 Lumbar Vertebrae
Those numbers come from populations where the variation was strictly defined. In a study of asymptomatic Chinese volunteers, the rates were notably higher: 5.5% had six lumbar vertebrae, and 5.8% had only eleven thoracic vertebrae (instead of the usual twelve). Overall, about one in ten participants in that study had an atypical count somewhere in the thoracolumbar region.3PubMed Central. Rate of presence of 11 thoracic vertebrae and 6 lumbar vertebrae in asymptomatic Chinese adult volunteers The discrepancy between studies reflects differences in imaging methods, populations, and how strictly researchers draw the line between “a transitional vertebra” and “a genuinely extra segment.” But the takeaway is consistent: having exactly five lumbar vertebrae is the overwhelming norm, yet departures from it are far from rare.
Lumbosacral Transitional Vertebrae
The most common reason someone ends up with an apparent four or six lumbar vertebrae is a lumbosacral transitional vertebra, or LSTV. This is a vertebra sitting at the boundary between the lumbar spine and the sacrum that has features of both regions. It comes in two flavors. In sacralization, the lowest lumbar vertebra (usually L5) partially or fully fuses with the sacrum, effectively reducing the mobile lumbar count to four. In lumbarization, the uppermost sacral segment separates from the sacrum and takes on lumbar characteristics, giving the appearance of six lumbar vertebrae.4PubMed Central. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance5PubMed. Analysis of Spinopelvic Parameters and Lumbar Lordosis in Patients with Transitional Lumbosacral Vertebrae, with Special Reference to Sacralization and Lumbarization
An LSTV is not always a clean binary. The transitional vertebra may have an enlarged transverse process on one or both sides that forms a joint-like connection or even a bony bridge to the sacrum, while the rest of the vertebra still looks lumbar. Radiologists classify these on a spectrum from mild enlargement of the transverse process all the way to complete fusion with the sacrum. The practical result is that two radiologists looking at the same scan can disagree about whether a given person has “five lumbar vertebrae with a transitional L5” or “four lumbar vertebrae with a sacralized segment.” That ambiguity is not just academic: it has direct consequences in the operating room.
Why the Count Matters in Surgery and Pain Management
Spine surgeons identify a problem disc or vertebra by its number: L4-L5, L5-S1, and so on. If the surgeon assumes five lumbar vertebrae but the patient actually has six (or four), the numbering of every segment shifts, and an operation planned for the L4-L5 disc could end up at the wrong level entirely. Wrong-level spine surgery is a recognized complication, and anatomical variation in the lumbosacral region is one of its major risk factors.6PubMed Central. Anatomical Variations That Can Lead to Spine Surgery at the Wrong Level: Part III Lumbosacral Spine The stakes are high enough that imaging protocols and counting methods have become a subject of research in their own right.
Beyond surgical planning, having a transitional vertebra changes how your lower back ages. The disc just above a transitional vertebra tends to degenerate faster than discs at other levels, while the disc between the transitional vertebra and the sacrum tends to be better preserved.7PubMed. Intervertebral disc degeneration associated with lumbosacral transitional vertebrae: a clinical and anatomical study The likely explanation is that when the lowest segment is partly fused to the sacrum, the segment above it has to absorb more motion and stress than it was designed for. Over time, that extra workload accelerates wear and tear. A more recent cross-sectional study confirmed this pattern, identifying LSTV as a risk factor for progressive disc degeneration at the adjacent level.8PubMed Central. Presence of sacralized lumbar vertebra predisposes to adjacent level lumbar disc degeneration: A cross-sectional study
The same mechanical logic applies to spinal stenosis, the narrowing of the spinal canal that can compress nerves. When one segment is immobilized by fusion with the sacrum, the increased mobility and abnormal forces at the level above can lead to degenerative changes in the joints and ligaments that eventually narrow the canal.9PubMed Central. Is Lumbosacral Transitional Vertebra Associated with Degenerative Lumbar Spinal Stenosis?
Bertolotti’s Syndrome
When a transitional vertebra actually causes chronic low back pain, the condition is called Bertolotti’s syndrome, named after the Italian radiologist who described it in 1917.10PubMed Central. A Comprehensive Update of the Treatment and Management of Bertolotti’s Syndrome: A Best Practices Review The pain mechanism involves the “false joint” that forms when an enlarged transverse process presses against the sacrum without fully fusing. That pseudo-articulation is prone to arthritic changes and can develop bone spurs that pinch nearby nerve roots.11PubMed Central. A Review of Symptomatic Lumbosacral Transitional Vertebrae: Bertolotti’s Syndrome
Bertolotti’s syndrome is something of a diagnosis of exclusion. Many people with transitional vertebrae never have symptoms, so finding one on a scan doesn’t automatically explain someone’s back pain. Doctors typically confirm it by injecting a local anesthetic into the pseudo-joint; if the pain goes away, the transitional vertebra is the likely culprit. Treatment ranges from physical therapy and steroid injections to surgical resection of the enlarged transverse process in stubborn cases.
How Doctors Count Vertebrae Accurately
If a person’s lumbar count can vary, how do radiologists figure out which vertebra is which? The gold standard is whole-spine imaging, starting from C2 (the second cervical vertebra, which has a distinctive shape that makes it easy to identify) and counting downward through every segment.12PubMed. A review of lumbosacral transitional vertebrae and associated vertebral numeration In practice, many lumbar MRI scans only capture the lower spine, which means the radiologist is often starting from the bottom and working up, using landmarks like the sacrum, the last rib-bearing vertebra, or the iliac crests to guess where they are. That approach is faster but error-prone when the anatomy is unusual.
One practical improvement is cross-referencing. If a radiology system stores both a cervicothoracic and a lumbosacral scan for the same patient, the radiologist can link them and count consistently from C2 all the way down, even without a single full-spine image.13PubMed. Numbering of vertebrae on MRI using a PACS cross-referencing tool This is especially important before any planned procedure. If you’re told you need surgery on a specific lumbar disc, it’s reasonable to ask whether your surgeon has confirmed the vertebral numbering with a complete count from the top of the spine.
What Drives the Variation
The boundaries between spinal regions are set during embryonic development by a family of genes called HOX genes. These genes act like a molecular address system, telling each developing vertebra whether it should become cervical, thoracic, lumbar, or sacral. Small shifts in when and where these genes activate can push the boundary between the lumbar spine and the sacrum one segment up or down, producing sacralization or lumbarization.14Nature Communications. HOX gene expression in the developing human spine The vertebral column in mammals is broadly variable across species, shaped by adaptation to different body plans and lifestyles.15PubMed Central. Fast running restricts evolutionary change of the vertebral column in mammals
These shifts are not random errors. They’re variations in a developmental program that has been tuned over millions of years and occasionally settles on slightly different outcomes in different individuals. Most of the time the result is completely benign: a person with six lumbar vertebrae and eleven thoracic vertebrae still has the same total number of mobile vertebrae, and their spine functions normally.
Congenital Vertebral Anomalies in Children
Numerical variation in healthy adults is distinct from congenital vertebral anomalies that can cause problems in childhood. A hemivertebra, for example, is a wedge-shaped vertebra that forms when only one side of a developing vertebral body grows properly. It doesn’t change the lumbar count in the usual sense, but it adds asymmetry that can lead to scoliosis or kyphosis. If left untreated, these deformities tend to progress as the child grows, and late correction requires longer surgical fusions with greater risk of neurological complications.16International Journal of Surgery Case Reports. Scoliosis secondary to neglected Hemivertebra: A case report Early detection and intervention produce better outcomes, which is one reason pediatric screening for spinal deformity exists.
An extra vertebra at the thoracolumbar junction (a vertebra with features intermediate between the last thoracic and first lumbar segments) is another recognized anomaly. Such cases have turned up in forensic contexts, where an unexpected extra bone in a set of skeletal remains could lead an examiner to incorrectly conclude the remains belong to more than one person.17Legal Medicine. An autopsy case of human skeletal remains with a numerical variation in thoraco-lumbar vertebrae The forensic takeaway is the same as the surgical one: always count from the top, and don’t assume the textbook number.
Did Our Ancestors Have More Lumbar Vertebrae?
For decades, the conventional view in paleoanthropology was that early hominids like the australopiths had six lumbar vertebrae, and that the human lineage lost one on the way to our current five. This idea traces back to interpretations of a single partial skeleton of Australopithecus africanus from Sterkfontein, South Africa. But a detailed re-analysis of that skeleton and comparisons with other early hominin fossils found that the evidence for six lumbar vertebrae was weaker than assumed.18PubMed. Vertebrae numbers of the early hominid lumbar spine The authors argued the most straightforward reading of the fossils is that early hominids had the same spinal formula as modern humans.
The debate is not settled. Other researchers have maintained that the modal number for australopiths and early Homo was indeed six, with the reduction to five occurring later in human evolution.19Journal of Human Evolution. Functional implications of variation in lumbar vertebral count among hominins A more recent analysis of vertebral evolution across primates supports what is called the “short-back” model: the ancestor humans share with chimpanzees likely had an African-ape-like vertebral column with a short lumbar region (possibly only four lumbar vertebrae), and the human five-segment lumbar spine represents a slight lengthening from that starting point rather than a shortening from a six-segment ancestor.20Journal of Human Evolution. Evolution of vertebral numbers in primates, with a focus on hominoids and the last common ancestor of hominins and panins
This matters beyond academic curiosity because it reframes the variation we see in living humans. If five lumbar vertebrae represent a derived condition that emerged relatively recently in evolutionary terms, the occasional appearance of four or six segments in modern people isn’t a defect but an echo of the flexibility that has always existed at this developmental boundary. The lumbosacral junction has been a zone of transition for millions of years, and it remains one today.
Living With a Non-Standard Lumbar Count
If you’ve had a spinal MRI and been told you have a transitional vertebra, or four or six lumbar segments, the most important thing to know is that this finding alone doesn’t mean you’ll have problems. Most people with numerical variants are completely asymptomatic. The variant only becomes clinically relevant if it contributes to pain (as in Bertolotti’s syndrome), accelerates disc degeneration at an adjacent level, or complicates surgical planning.
What you should do is make sure the finding is documented clearly in your medical records, with the counting method noted. If you ever need spinal imaging or a procedure in the future, having that baseline prevents the kind of numbering confusion that leads to errors. Ask whether your vertebrae were counted from C2 down on a whole-spine or cross-referenced image rather than assumed from a limited lumbar scan. That single piece of information can save a lot of trouble down the line.