Sacrum pain: what hurts at the base of your spine

Pain at the very base of the spine, below the belt line and above the tailbone, often gets lumped in with ordinary lower back pain. Patients often describe it simply as “low back pain,” when the pain is really centred somewhere more specific. It usually isn’t the same thing. The sacrum is a distinct bone with its own joints, its own blood supply, and its own set of problems, and treating sacrum pain as generic back pain can mean missing the actual cause for months.

This article walks through what the sacrum does, the handful of conditions that commonly cause pain there, the rarer ones worth knowing about, and the pattern of symptoms that means a same-day assessment rather than a wait-and-see approach. It is also one of the areas where the right diagnosis changes the treatment plan the most, since a joint problem, a stress fracture and a referred pelvic cause are managed in genuinely different ways, even though all three can feel similar at first.

Where the sacrum sits and what it does

The sacrum is a triangular, slightly curved bone formed from five fused vertebrae, sitting directly below the lumbar spine and directly above the tailbone, or coccyx. It forms the back wall of the pelvis and connects to the two hip bones through the sacroiliac joints, one on each side.

Unlike the vertebrae above it, the sacrum has almost no independent movement of its own. Its job is structural: it transfers load from the upper body, through the spine, down into the pelvis and legs, and it anchors several of the ligaments that keep the pelvis stable during walking, standing and lifting.

Because the sacrum sits at this transfer point, pain there can come from the bone itself, from the joints on either side of it, from the nerves that pass through it, or from pelvic structures nearby that refer pain upward. Working out which of these is involved is the real diagnostic task.

The sacroiliac joints: the most common source

The sacroiliac joints are the most frequent source of pain in this region, and sacroiliac joint dysfunction accounts for a substantial share of presentations that patients describe as sacrum pain. These joints have very little natural movement, but they do need to tolerate small shifts and rotational forces with every step.

Pregnancy is a well-recognised trigger, since hormonal changes loosen the ligaments supporting these joints to allow the pelvis to accommodate childbirth, and that same looseness can leave the joints less stable for months afterward. A fall directly onto the buttocks, a sudden twist while lifting, or a long period of asymmetric loading, such as always standing with weight through one leg, can produce the same irritation.

The pain typically sits to one side of the midline, just below the belt line, and can radiate into the buttock or the back of the thigh, which sometimes leads to it being mistaken for sciatica. It is usually worse with movements that load the joint asymmetrically, such as climbing stairs, rolling over in bed, or standing up from sitting.

Sacral stress fractures

A stress fracture in the sacrum is a genuine, if less commonly recognised, cause of pain in this area. It happens when repetitive loading outpaces the bone’s ability to repair itself, and it is seen most often in long-distance runners, in postmenopausal women with reduced bone density, and in anyone who has had radiation treatment to the pelvis, which can weaken the bone over time.

Unlike a traumatic fracture from a single fall, a stress fracture can develop gradually, with pain that builds over days or weeks of increasing activity rather than appearing suddenly. It often worsens with weight-bearing and standing, and can be difficult to localise precisely, which contributes to delayed diagnosis.

Plain X-rays frequently miss sacral stress fractures in their early stages, because the fracture line can be subtle against the complex shape of the bone. An MRI is considerably more sensitive and is usually the next step when a stress fracture is suspected but not visible on X-ray, particularly in a runner or in someone with known low bone density.

Other bone and nerve causes

Beyond the joints and stress fractures, a smaller group of conditions involve the sacral bone or the nerves running through it. Tarlov cysts, fluid-filled sacs that form around the nerve roots within the sacrum, are a known but often incidental finding; many cause no symptoms at all, but a subset do produce localised pain, and occasionally numbness or bladder symptoms, when they are large enough to press on nearby nerves.

Sacral insufficiency fractures, a specific form of stress fracture driven primarily by weakened bone rather than unusually high activity, are more common in older adults with osteoporosis and can occur with minimal or no clear trigger. These are sometimes discovered only after a scan ordered for a different reason.

Rarely, a bone tumour, either arising in the sacrum itself or spreading there from elsewhere, can present as persistent sacral pain, generally following a pattern of being present at rest and at night rather than only with activity. This is uncommon, but it is one of the reasons unexplained, persistent sacral pain in someone without an obvious mechanical cause deserves proper imaging rather than being assumed to be muscular.

Gynaecological and pelvic causes that mimic sacrum pain

Because the sacrum sits at the back of the pelvis, several gynaecological and pelvic conditions can refer pain to this area in a way that feels, to the patient, exactly like a spine problem. Endometriosis, certain ovarian conditions, and pelvic inflammatory processes have all been documented to produce a deep ache centred over the sacrum, sometimes without significant pain elsewhere in the pelvis.

This referred pattern matters clinically because it means a sacrum-pain presentation that does not respond to the usual joint- or bone-directed treatment, or that comes with other pelvic symptoms such as abnormal bleeding, pain with intercourse, or a clear relationship to the menstrual cycle, is worth discussing with a gynaecologist alongside a spine assessment rather than only pursuing one line of investigation.

The reverse also happens: pelvic floor dysfunction, where the muscles supporting the pelvic organs become overly tense or poorly coordinated, can produce pain that is felt at the sacrum and is genuinely a musculoskeletal problem, just one centred on the pelvic floor rather than the sacroiliac joints themselves. This is treated differently, usually with specialised pelvic floor physiotherapy.

Everyday habits that make sacroiliac pain worse

A handful of ordinary daily patterns tend to aggravate sacroiliac joint pain specifically, and recognising them often does more for day-to-day comfort than any single treatment. Standing with weight shifted onto one leg, a habit many people fall into without noticing, loads one sacroiliac joint more than the other and can keep an irritated joint from settling.

Sitting with a wallet, phone, or anything else in a back pocket on one side has the same effect while seated, tilting the pelvis slightly and loading one joint more than the other over the course of a long day. Low, soft sofas that let the hips sink below the knees also tend to aggravate sacroiliac pain, since getting up from that position asks the joint to do more work than a firmer, higher seat would.

Activities that combine bending with twisting, such as lifting a child out of a car seat or reaching into the back seat of a car from an awkward angle, are a particularly common trigger, because they load the sacroiliac joint asymmetrically at the exact moment it is least able to resist that load. Learning to square the body toward whatever is being lifted, rather than twisting through the pelvis, is one of the more effective everyday adjustments.

High-impact exercise, particularly running on hard surfaces or sports involving sudden changes of direction, can also aggravate a sacroiliac joint that has not yet regained its normal stability, even once the original pain has mostly settled. This is why a graded return to activity, rather than resuming a previous training load immediately, matters for this particular joint more than it does for some other causes of back pain.

How sacrum pain is diagnosed

The starting point is always a careful history: where exactly the pain is felt, what specific movements or positions reproduce it, whether there was a triggering injury, and whether any bladder, bowel or gynaecological symptoms accompany it. This history alone often narrows the likely cause considerably before any scan is ordered.

A physical examination follows, including specific tests that stress the sacroiliac joints in particular directions to see if they reproduce the pain, along with an assessment of the lower spine to rule out a disc or nerve-root cause that happens to refer pain into the same general area.

Imaging is chosen based on what the history and examination suggest rather than ordered as a blanket first step. An X-ray is a reasonable starting point for suspected trauma or significant bone concern, but an MRI is usually needed to properly evaluate the sacroiliac joints, to find a stress fracture that an X-ray has missed, or to look for a Tarlov cyst or other soft-tissue finding.

Treatment and when to see a doctor

Treatment follows the diagnosis rather than a generic approach, which is exactly why getting the diagnosis right matters so much here. Sacroiliac joint dysfunction typically responds to targeted physiotherapy aimed at restoring stability around the joint, sometimes alongside a period of activity modification, and a diagnostic or therapeutic injection into the joint if conservative measures alone are not enough.

A confirmed stress fracture usually requires a period of reduced weight-bearing activity, addressing any underlying bone density issue if one is found, and a gradual, supervised return to loading rather than pushing through the pain. A Tarlov cyst that is genuinely symptomatic, and other rarer bone or nerve causes, are managed on a case-by-case basis once properly identified.

Sacrum pain accompanied by new numbness in the saddle area, loss of bladder or bowel control, fever, or pain that appeared without any clear trigger and continues to worsen should prompt a same-day assessment rather than a trial of rest, since these particular combinations point toward causes that genuinely should not wait. For everything else, a proper diagnosis first, rather than guessing at treatment, is what actually shortens the time to feeling better.

Outlook: how long sacrum pain typically takes to settle

Most sacroiliac joint pain genuinely improves with the right combination of activity modification and targeted physiotherapy, usually over a period of six to twelve weeks, though mild flare-ups can recur for longer, particularly in the months after pregnancy or a significant fall. Consistency with the exercises a physiotherapist prescribes tends to matter more for this particular joint than for many other spine complaints, because the goal is retraining stability rather than simply waiting for inflammation to pass.

A sacral stress fracture takes longer, typically six to twelve weeks of reduced loading before a gradual return to full activity is appropriate, and the timeline depends heavily on whether an underlying bone density issue is identified and addressed alongside the fracture itself. Rushing back to running or high-impact activity before the bone has had time to heal is the most common reason a stress fracture takes longer than expected to resolve fully.

For the rarer causes, a Tarlov cyst that is genuinely producing symptoms, a tumour, or a significant pelvic condition referring pain to the sacrum, the timeline depends entirely on the specific diagnosis and the treatment it requires, which is one more reason an accurate diagnosis early on matters more than trying a string of generic treatments and waiting to see what helps.

Whatever the cause turns out to be, persistent sacrum pain that has not meaningfully improved after several weeks of appropriate treatment, or pain that changes character, becoming worse at night or at rest rather than with activity, is a reasonable point to ask for a review of the diagnosis rather than simply continuing the same plan for longer.