Bony pelvis
Bony pelvis: types, inlet and outlet diameters and their obstetric importance
The fetal head must pass three planes of the true pelvis (inlet, mid-cavity and outlet), each with a different longest diameter, so the head enters transversely and leaves anteroposteriorly. CPSP asks the conjugates and their measurements, the four pelvic shapes and the labour problem each causes, and the bony landmarks used in labour and regional blocks.
False and true pelvis
- The pelvic brim (inlet) divides the false pelvis above (part of the abdomen) from the true pelvis below.
- Brim, from behind forwards: sacral promontory (front upper edge of the body of S1), ala of the sacrum, arcuate line of the ilium, pectineal line, pubic crest, upper border of the symphysis. The iliac crest is not part of it.
- Structures crossing the brim: ureter (over the common iliac bifurcation), ovarian vessels in the infundibulopelvic ligament, the round ligament. Fluid poured in at hysterectomy that passes the brim runs into the paracolic gutters.
- Inclination: standing, the inlet plane is tilted about 55–60 degrees to the horizontal (the outlet about 10–15 degrees); a high inclination delays engagement.
Joints
- Symphysis pubis: secondary cartilaginous joint (fibrocartilage disc), divided in symphysiotomy; widens slightly in pregnancy.
- Sacroiliac joint: synovial plane joint with strong interosseous ligaments; nutation alters pelvic diameters. Pregnancy hormones (relaxin, progesterone) loosen both joints, causing pelvic girdle pain.
- Sacrococcygeal joint: the coccyx moves back about 2 cm at crowning, enlarging the outlet; strain causes coccydynia, and a fused forward-angled coccyx narrows the outlet.
Diameters of the planes
| Plane | Anteroposterior | Transverse | Oblique | Head position |
|---|---|---|---|---|
| Inlet | Obstetric conjugate 11 cm (shortest) | 13 cm (longest) | 12 cm | Enters transverse or oblique |
| Mid-cavity (greatest dimensions) | 12 cm | 12 cm | 12 cm | Rotates |
| Mid-pelvis (least dimensions) | 11.5–12 cm | Interspinous 10 cm (narrowest in the pelvis) | – | Arrest here: deep transverse arrest |
| Outlet | 13 cm (with coccyx back, longest) | Intertuberous 11 cm | – | Leaves anteroposterior, occiput under the symphysis |
- Oblique diameters are named by the sacroiliac joint they start from; in left occipitoanterior the sagittal suture lies in the right oblique.
- Outlet boundaries: pubic arch in front, ischial tuberosities laterally, sacrotuberous ligaments posterolaterally, tip of the coccyx behind.
The three conjugates
| Conjugate | From promontory to | Length | Use |
|---|---|---|---|
| True (anatomical) | Upper border of symphysis | 11 cm | Anatomical only |
| Obstetric | Nearest point on the back of the symphysis | About 10.5–11 cm | Shortest AP space the head must pass |
| Diagonal | Lower border of symphysis | About 12.5 cm | Measured on vaginal examination |
- Obstetric conjugate is about the diagonal minus 1.5–2 cm. If the promontory is reached easily the inlet is contracted (or L5 has slipped forward in spondylolisthesis and is mistaken for it).
- Clinical outlet test: four knuckles between the ischial tuberosities (intertuberous diameter).
Pelvic types
| Type | Inlet | Features | Labour effect |
|---|---|---|---|
| Gynaecoid (about 50%) | Round | Wide subpubic angle (about 90 degrees), blunt spines | Most favourable |
| Android | Heart-shaped | Converging side walls, prominent spines, narrow subpubic angle | Deep transverse arrest, mid-cavity and outlet trouble |
| Anthropoid | Long oval, AP longer | Long narrow sacrum | Engages direct OA or OP; may deliver face to pubes |
| Platypelloid | Flat, transverse longer | Short AP inlet | Enters transverse with asynclitism; once past the brim, usually progresses |
- Rickets (childhood) pushes the promontory forward: short AP inlet, flat pelvis. Osteomalacia (adult) gives a triradiate pelvis with beaked symphysis and very narrow arch.
- Asynclitism: lateral tilt of the head, named by the parietal bone that leads; anterior asynclitism (Naegele obliquity) helps entry into a flat pelvis.
Fetal skull diameters
| Attitude | Presenting diameter | Length |
|---|---|---|
| Well-flexed vertex (OA) | Suboccipitobregmatic | 9.5 cm |
| Deflexed vertex (OP) | Occipitofrontal | 11.5 cm |
| Brow | Mentovertical | 13.5 cm, cannot deliver vaginally at term |
| Face | Submentobregmatic | 9.5 cm |
- The biparietal diameter (9.5 cm) is the same in all attitudes. Moulding, overlapping of unfused vault bones, reduces diameters; severe moulding suggests disproportion.
- Shoulder dystocia: the anterior shoulder impacts behind the symphysis; McRoberts hyperflexion straightens the lumbosacral angle and rotates the symphysis upward without enlarging bony diameters.
Landmarks in labour
- Engagement: biparietal diameter through the brim; abdominally two-fifths or less palpable. A primigravida's head is usually engaged by 37–38 weeks; a high head that overrides the symphysis suggests inlet disproportion.
- Station 0 is the level of the ischial spines; minus above, plus below.
- The ischial spine is the landmark for pudendal nerve block and the narrowest transverse point.
Remember The inlet is widest side to side, the outlet widest front to back, so the head enters transverse and rotates to deliver anteroposterior.
Exam trap The obstetric conjugate, not the true conjugate, is the shortest AP diameter of the inlet; the diagonal conjugate is the one you can measure.
Exam trap The interspinous diameter (10 cm) is the narrowest in the pelvis; prominent spines with a narrow arch mean android pelvis and deep transverse arrest.
Test yourself on this topic
5 of the 39 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A well-flexed head in the occipitoanterior position presents the smallest possible anteroposterior skull diameter to the pelvis. Which diameter is it?
Answer: Suboccipitobregmatic
With full flexion the chin is on the chest, and the presenting diameter runs from below the occipital protuberance to the centre of the anterior fontanelle: the suboccipitobregmatic, about 9.5 cm. This, with the biparietal diameter of 9.5 cm, makes a round, small presenting circle that fits the pelvis well. Deflexion brings larger diameters into play, slowing labour.
Flexed head
- Suboccipitobregmatic 9.5 cm
- With biparietal 9.5 cm
- Smallest presenting circle
- Favoured by OA position
Remember Flexion makes the head smallest.
Exam trap The biparietal diameter is the same in all attitudes; flexion changes only the anteroposterior one.
Why the other options are wrong
- Occipitofrontal About 11.5 cm; presents when the head is deflexed.
- Mentovertical About 13.5 cm; presents in brow.
- Submentovertical About 11.5 cm; occurs with partial extension.
- Bitemporal A transverse diameter of about 8 cm, not anteroposterior.
In a woman standing upright, the plane of the pelvic inlet is tilted forwards rather than horizontal. Approximately what angle does it make with the horizontal?
Answer: About 55–60 degrees
In the erect posture the pelvis is tilted so that the anterior superior iliac spines and the pubic tubercles lie in roughly the same vertical plane, and the plane of the inlet makes about 55–60 degrees with the horizontal. The plane of the outlet is tilted about 10–15 degrees. A high inclination can delay engagement, and this tilt is why the uterus must direct the head backwards into the brim.
Pelvic inclination
- Inlet plane 55–60° to horizontal
- Outlet plane 10–15°
- ASIS and pubic tubercle in one vertical plane
- High inclination delays engagement
Remember The inlet faces forwards and upwards.
Exam trap Do not picture the inlet as horizontal; the head must enter it downwards and backwards.
Why the other options are wrong
- About 10–15 degrees This is roughly the tilt of the outlet plane.
- About 30 degrees Too small for the inlet in the erect position.
- About 90 degrees That would make the inlet vertical.
- About 120 degrees Beyond vertical; not anatomically possible here.
A primigravida has a long, painful labour with backache. Examination finds the anterior fontanelle easily felt near the centre of the pelvis and the occiput towards the right sacroiliac joint. Why is descent slower than with an occipitoanterior head?
Answer: A larger occipitofrontal diameter presents
In the occipitoposterior position the head is often deflexed, because the fetal spine lies against the maternal spine. Instead of the 9.5 cm suboccipitobregmatic diameter, the larger occipitofrontal diameter (about 11.5 cm) presents, so engagement and descent are slower and the occiput has to rotate further (three-eighths of a circle) to come anterior. A palpable anterior fontanelle near the centre indicates deflexion.
Occipitoposterior position
- Head deflexed
- Occipitofrontal 11.5 cm presents
- Long rotation: 3/8 of a circle
- Backache, slow labour
Remember OP: deflexed head, bigger diameter, slow labour.
Exam trap Most OP heads rotate anteriorly by themselves with good contractions.
Why the other options are wrong
- The mentovertical diameter presents That is a brow presentation.
- The biparietal diameter increases The biparietal diameter does not change.
- The head enters in the anteroposterior plane Most OP heads engage obliquely.
- The submentobregmatic diameter presents That is a face presentation.
In symphysiotomy, sometimes done where caesarean section is not possible, the surgeon divides the fibrocartilaginous disc between the two pubic bones. What type of joint is being divided?
Answer: Secondary cartilaginous joint
The symphysis pubis is a secondary cartilaginous joint (symphysis): the articular surfaces are covered with hyaline cartilage and joined by a fibrocartilaginous disc, reinforced by the superior and arcuate pubic ligaments. Such joints lie in the midline and allow slight movement. Primary cartilaginous joints (synchondroses) are hyaline only, such as epiphyseal plates, and the sacroiliac joint is largely synovial.
Symphysis pubis
- Secondary cartilaginous joint
- Hyaline cartilage + fibrocartilage disc
- Arcuate pubic ligament below
- Widens slightly in pregnancy
Remember Midline symphyses are secondary cartilaginous joints.
Exam trap Intervertebral discs and the symphysis pubis are both secondary cartilaginous joints.
Why the other options are wrong
- Primary cartilaginous joint Hyaline cartilage only, e.g. growth plates.
- Synovial plane joint Describes the sacroiliac joint, not the symphysis.
- Fibrous syndesmosis Joined by ligament, e.g. inferior tibiofibular joint.
- Fibrous suture Found between skull bones.
While tracing the margin of the pelvic inlet on a skeleton, a student follows the arcuate line of the ilium forwards along the pectineal line. Which structure forms the next part of the brim before the symphysis?
Answer: Pubic crest
The pelvic brim (inlet) is formed, from behind forwards, by the sacral promontory, the ala of the sacrum, the arcuate line of the ilium, the pectineal line of the pubis, the pubic crest and the upper border of the symphysis pubis. The iliac crest and inguinal ligament lie above and in front of the false pelvis, and the obturator crest is on the margin of the obturator foramen.
Pelvic brim
- Sacral promontory and ala
- Arcuate line of ilium
- Pectineal line and pubic crest
- Upper border of symphysis
Remember Promontory, ala, arcuate, pectineal, crest, symphysis.
Exam trap The iliac crest belongs to the false pelvis, not the brim.
Why the other options are wrong
- Ischial spine The ischial spine lies on the side wall of the lesser pelvis, not on the brim.
- Iliac crest Part of the false pelvis, well above the brim.
- Inguinal ligament Soft tissue in front of the hip, not part of the brim.
- Obturator crest Lies on the margin of the obturator foramen.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Pelvis, pelvic floor and perineum, Bony pelvis: types, inlet and outlet diameters and their obstetric importance