Appearance
5.6 — The Pelvis, Male and Female
The pelvis is the only region of the skeleton where the differences between male and female bodies are large enough to identify a skeleton with about 95 percent accuracy from that one structure. The reason is not decoration or hormones acting broadly — it is that one of the two pelvises has to let a head with a 10 centimetre diameter pass through it.
Everything about female pelvic anatomy in this chapter follows from that requirement, and Chapter 3.6 explained what it cost.
The structure
The pelvis is a ring made of four bones: two hip bones (each itself three fused bones), the sacrum, and the coccyx.
Each hip bone is three bones fused. They are separate in childhood and fuse at around 15 to 17 years, meeting at the hip socket.
Ilium — the large flared blade at the top. Its upper rim is the iliac crest, which you feel when you put your hands on your hips. Ischium — the lower back part. Its thickened ischial tuberosity is what you sit on. Pubis — the lower front part. The two pubic bones meet in the midline at the pubic symphysis, a joint of fibrocartilage.
All three meet at the acetabulum, the deep cup that receives the head of the femur.
The sacroiliac joints at the back connect the sacrum to each ilium. These are extremely strong and barely move — a few degrees at most — and they have to be, because your entire upper body weight passes through them into the legs.
So the pelvis is a closed ring, and that has a direct clinical consequence: a ring cannot break in only one place. If you see one fracture in a pelvic ring on an X-ray, look for the second, because the ring must have failed somewhere else too, either as another fracture or as a disrupted joint.
What the pelvis does
It transmits weight. The load from the spine passes into the sacrum, across the sacroiliac joints into the ilia, and down through the acetabula into the femurs. Standing on one leg, the entire body weight passes through one sacroiliac joint and one hip.
It anchors muscle. The largest muscles in the body attach here — the gluteals behind, the hip flexors in front, the hamstrings from the ischial tuberosity, and the abdominal wall along the iliac crest.
It supports the pelvic organs. The bladder, rectum and reproductive organs sit within the ring, held by the pelvic floor — a sling of muscle across the bottom of the pelvis with openings for the urethra, the anus, and in females the vagina.
And it protects. The pelvic bones shield the organs within them, but this protection has a cost covered below.
Male and female pelvis compared
| Feature | Male | Female |
|---|---|---|
| Overall shape | Narrow, deep | Wide, shallow |
| Pelvic inlet | Heart-shaped | Oval, wider side to side |
| Subpubic angle | 50–60° (narrow V) | 80–90° (wide U) |
| Sacrum | Long, narrow, curved | Short, wide, flatter |
| Ischial spines | Project inward | Less prominent |
| Acetabula | Larger, closer together | Smaller, further apart |
| Bone | Heavier, rougher | Lighter, smoother |
The subpubic angle is the single most reliable feature, and it is the one a forensic anthropologist checks first. It can be estimated on a living person by placing the index and middle fingers under the pubic bones.
Every difference above enlarges the birth canal. Wider inlet, flatter sacrum, less inward-projecting spines, wider subpubic arch — all of them increase the space a head must pass through.
And they are not free. The acetabula being further apart means the femurs angle inward more sharply to bring the knees under the body's centre line. That greater angle at the knee is one reason anterior cruciate ligament injuries are two to eight times more common in female athletes in sports involving pivoting and landing, alongside differences in muscle activation patterns and ligament laxity. Chapter 5.7.
The changes appear at puberty, driven by oestrogen. Before it, male and female pelvises are hard to distinguish.
The birth canal, and why labour is a three-stage rotation
The passage a baby must negotiate is not a straight tube, and its shape changes along its length. This is the anatomical reason childbirth takes hours rather than minutes.
The pelvic inlet — the entrance at the top — is widest side to side. Roughly 13 cm transverse against 11 cm front to back.
The mid-cavity is roughly circular, and it is where the ischial spines project inward, making this the narrowest point in many pelvises.
The pelvic outlet — the exit at the bottom — is widest front to back. Roughly 12.5 cm front to back against 11 cm transverse.
So the widest axis rotates 90 degrees between entry and exit, and the fetal head, which is itself oval, must rotate to match. It enters facing sideways, rotates to face backward as it descends, and then extends to emerge.
No other primate's infant has to do this, and it is why human birth normally needs assistance while a chimpanzee gives birth alone (Chapter 3.6).
Four pelvic shapes are classically described — gynaecoid (the typical female round-oval, most favourable), android (male-like, heart-shaped, more likely to obstruct), anthropoid (oval front to back), and platypelloid (flattened). In practice these categories are of limited predictive value, and modern obstetrics relies on progress in labour rather than on measuring the pelvis in advance, because a pelvis that looks unfavourable often delivers normally and the reverse.
The joints soften in pregnancy. The hormone relaxin loosens the ligaments of the pubic symphysis and sacroiliac joints, allowing the pelvis to widen by a few millimetres during delivery. The cost is pelvic girdle pain, which affects a substantial minority of pregnant women and can be severe, and instability that persists for some months after delivery.
Obstructed labour — where the head simply cannot pass — was uniformly fatal to both mother and baby before caesarean section. It remains a leading cause of maternal death where surgical care is unavailable, and its non-fatal consequence is often worse than death is treated as: prolonged pressure of the head against the pelvis kills the tissue between the vagina and the bladder or rectum, producing an obstetric fistula with continuous incontinence. It is repairable surgically, and hundreds of thousands of women live with it untreated.
Pelvic fractures
A pelvic fracture from high-energy trauma is one of the most dangerous injuries in medicine, and the danger is bleeding.
The pelvis is surrounded by a dense network of veins and arteries, including branches of the internal iliac artery running directly against bone. A disrupted pelvic ring tears them, and the pelvic cavity can accommodate several litres of blood — with nothing to compress it against, so bleeding does not stop by itself.
Mortality is around 10 to 20 percent overall, and considerably higher for the most unstable patterns.
The pre-hospital treatment is a pelvic binder — a wide strap applied at the level of the greater trochanters, tightened to close the ring. This works for a mechanical reason worth stating: reducing the pelvic volume reduces the space available for blood to fill, and closing the fracture surfaces allows clot to form. It is applied before imaging, because the diagnosis is suspected clinically and the intervention is harmless if wrong.
A crucial practical point: do not "spring" the pelvis to test it. Repeatedly compressing an unstable pelvis to see if it moves dislodges clot and worsens bleeding. The old examination technique has been abandoned.
Fragility fractures of the pelvis are a different entity — low-energy, in the elderly with osteoporosis, often from a simple fall. They usually involve the pubic rami, bleed far less, and are managed with pain relief and mobilisation.
The hip joint
A deep ball-and-socket joint, and the anatomical opposite of the shoulder in every design decision.
The acetabulum is deep, enclosing more than half the femoral head, and deepened further by a rim of cartilage. The capsule is thick and reinforced by three of the strongest ligaments in the body. The result is a joint that is very stable and considerably less mobile than the shoulder.
Dislocation therefore requires enormous force, typically a dashboard injury in which the knee strikes and drives the femur backward out of the socket. It is an emergency, because the blood supply to the femoral head can be interrupted, and reduction within about six hours substantially reduces the risk of the head dying.
The blood supply of the femoral head is the key to hip fracture management, and it is worth understanding because it decides the operation.
Most of the femoral head's blood arrives from vessels that run up along the femoral neck, from a ring of arteries at its base. A small additional supply runs in the ligament to the head, and in adults it is negligible.
So a fracture through the neck — an intracapsular fracture — tears those vessels, and the head loses its blood supply. A fracture below the neck — extracapsular — leaves them intact.
This decides the operation, and the logic is clean. An extracapsular fracture is fixed with a screw and plate, because the head will heal. An intracapsular displaced fracture in an older patient is treated by replacing the head, because fixing a fragment that is going to die produces a failed operation and a second, larger one later.
Hip fracture is one of the most consequential events in older people's lives. Around 1 in 3 women and 1 in 8 men over 50 will sustain an osteoporotic fracture. Mortality within one year of a hip fracture is roughly 20 to 30 percent, and only about half of survivors regain their previous level of independence. Surgery within 24 to 48 hours measurably improves outcomes, which is why it is prioritised over almost everything else on an emergency list.
The pelvic floor
A sling of muscle closing the bottom of the pelvic ring, with openings for the urethra, anus and, in females, the vagina.
It supports the pelvic organs against gravity and against every rise in abdominal pressure — coughing, lifting, straining. It also provides voluntary control of the urethral and anal sphincters.
Damage is common and under-discussed. Childbirth stretches and can tear it; chronic straining, obesity, chronic cough and ageing weaken it.
The consequences are stress incontinence — leaking urine on coughing, sneezing or exercise — and prolapse, where pelvic organs descend. Around a third of women experience some degree of pelvic floor dysfunction, and it is very substantially underreported because people assume it is an inevitable consequence of childbirth or age.
Pelvic floor muscle training genuinely works. Supervised programmes cure or substantially improve stress incontinence in a large majority, and it is a first-line treatment with no side effects — which makes the fact that it is so often skipped in favour of surgery or pads a real failure of care. Chapter 15.7.
What the next page fixes
From the pelvis the load passes into the longest, strongest bones in the body, through a knee that is mechanically the most vulnerable major joint, into a foot with 26 bones arranged as a shock-absorbing arch. Chapter 5.7 covers the lower limb, why the knee is injured so much more often than the hip, and how the arch of the foot works.