2.3 Clinical Pelvimetry & Assessment of the Bony Pelvis
Key Takeaways
- The obstetric conjugate cannot be measured directly and is estimated as the diagonal conjugate minus 1.5 to 2 cm; 10 cm or more is adequate.
- A diagonal conjugate of 11.5 to 12 cm or greater indicates an adequate pelvic inlet, and an unreachable sacral promontory is presumptive evidence of adequacy.
- The interspinous (bispinous) diameter at the ischial spines is the narrowest diameter of the entire pelvis; below about 9.5 cm predisposes to deep transverse arrest.
- The gynecoid pelvis occurs in about half of women and is most favorable, while the android pelvis is associated with deep transverse arrest and operative birth.
- Cochrane evidence shows X-ray pelvimetry in cephalic presentation increases cesarean birth without improving perinatal outcome, so cephalopelvic disproportion remains a diagnosis made in labor.
Why the Bony Pelvis Still Gets Assessed
The AMCB task list includes "performs pelvimetry to evaluate the bony pelvis." Clinical pelvimetry is a physical-exam skill, usually performed at the initial prenatal visit or in early labor, that describes the architecture of the passage through which the fetus must travel. Its limitations are as examinable as its technique: pelvimetry describes a pelvis, it does not predict whether a given labor will end in vaginal birth. Cephalopelvic disproportion is a retrospective diagnosis made in labor, not a prenatal prophecy.
Osteology and the Two Pelves
The bony pelvis is formed by the two innominate bones (each the fusion of ilium, ischium, and pubis), the sacrum, and the coccyx, joined at the two sacroiliac joints and the pubic symphysis.
The linea terminalis (iliopectineal line) divides the pelvis into:
- False (greater) pelvis — above the linea terminalis, bounded by the iliac fossae. It supports the gravid uterus but has no obstetric significance for passage.
- True (lesser) pelvis — below the linea terminalis. This is the obstetric canal, described by three clinically relevant planes.
The Three Obstetric Planes and Their Diameters
| Plane | Boundaries | Diameters That Matter | Adequate Values |
|---|---|---|---|
| Inlet | Sacral promontory posteriorly, linea terminalis laterally, superior pubic symphysis anteriorly | Diagonal conjugate (promontory to inferior symphysis border — the only one measurable clinically); obstetric conjugate (promontory to the innermost posterior symphysis — the true limiting AP diameter); true conjugate / conjugata vera (promontory to superior symphysis border) | Diagonal conjugate ≥11.5–12 cm; obstetric conjugate ≥10 cm; transverse diameter ~13 cm |
| Midpelvis | Inferior symphysis, ischial spines, sacrum at S4–S5 | Interspinous (bispinous) diameter — the narrowest diameter of the entire pelvis | ≥10 cm; below about 9.5 cm suggests midpelvic contraction and predisposes to deep transverse arrest |
| Outlet | Inferior symphysis, ischial tuberosities, sacrococcygeal joint | Intertuberous (bituberous) diameter; subpubic (pubic) arch angle; mobility of the coccyx | Intertuberous ≥8–8.5 cm (roughly the width of a closed adult fist); subpubic angle ≥90°; coccyx mobile |
[!IMPORTANT] The obstetric conjugate cannot be measured directly. It is estimated as the diagonal conjugate minus 1.5 to 2 cm. A diagonal conjugate of 12 cm therefore implies an obstetric conjugate of about 10–10.5 cm, which is adequate.
Technique of Clinical Pelvimetry
- Explain the exam, obtain consent, and position the patient in lithotomy with the bladder empty.
- Diagonal conjugate: insert the index and middle fingers into the vagina and sweep upward and posteriorly toward the sacral promontory. If the promontory cannot be reached, the inlet is presumed adequate. If it is reached, mark the point where the radial border of the hand meets the inferior symphysis, withdraw, and measure from fingertip to mark.
- Sacrum and sacral curve: sweep the fingers down the anterior sacrum. A deep, hollow, well-curved sacrum is favorable; a straight or forward-inclined sacrum reduces the posterior space.
- Ischial spines: palpate laterally at the midpelvis. Blunt, non-prominent spines are favorable; sharp, prominent, encroaching spines narrow the interspinous diameter.
- Sacrosciatic (sacrospinous) notch: should admit two to three fingerbreadths; a narrow notch signals a contracted posterior pelvis.
- Subpubic arch: place two fingers beneath the symphysis. An arch admitting two fingers comfortably approximates ≥90°; a narrow, gothic arch pushes the presenting part posteriorly onto the perineum.
- Intertuberous diameter: measure externally between the ischial tuberosities with a closed fist or pelvimeter.
- Coccyx: press posteriorly; a mobile coccyx allows the outlet to expand in the second stage.
Caldwell-Moloy Pelvic Types
| Type | Approximate Frequency | Inlet Shape | Obstetric Implication |
|---|---|---|---|
| Gynecoid | ~50% | Round to slightly oval transversely | The classic "female" pelvis; wide subpubic arch, blunt spines, hollow sacrum. Most favorable for vaginal birth, usually with occiput anterior rotation |
| Android | ~20% | Heart-shaped / wedge, narrowing anteriorly | "Male" type; convergent side walls, prominent spines, narrow subpubic arch. Least favorable; associated with deep transverse arrest, occiput posterior persistence, and operative birth |
| Anthropoid | ~25% | Oval with a long anteroposterior and short transverse diameter | Deep pelvis; favors occiput posterior or direct occiput anterior birth; generally permits vaginal birth |
| Platypelloid | ~3% | Flat, wide transversely and short anteroposteriorly | Rarest; the head must engage in the transverse diameter and may arrest at the inlet; poorest prognosis for engagement |
Most pelves are mixed types, described by the posterior (inlet) segment first and the anterior segment second.
What the Evidence Says About Predictive Value
- X-ray pelvimetry is not recommended. Cochrane reviews of radiographic pelvimetry in cephalic presentations show increased cesarean birth without improved perinatal outcomes.
- Clinical pelvimetry has poor sensitivity and specificity for predicting cephalopelvic disproportion. Fetal head molding, asynclitism, maternal position change, and the dynamic softening of the pelvic joints all change the effective dimensions during labor.
- The correct use of pelvimetry is therefore descriptive and anticipatory: a narrow subpubic arch or prominent spines prompts the midwife to plan upright, asymmetric, and squatting positions, to anticipate a longer second stage, and to be alert for deep transverse arrest — not to schedule a cesarean.
- Never counsel a patient that her pelvis is "too small" on the basis of a prenatal exam. The only reliable test of a pelvis is a trial of labor.
During an initial prenatal examination the midwife can just reach the sacral promontory and measures a diagonal conjugate of 11 cm. The ischial spines are prominent, the sacrosciatic notch admits two fingerbreadths, and the subpubic arch admits two fingers with difficulty. How should the midwife interpret and act on these findings?
A pelvis is described as having a heart-shaped, wedge-like inlet that narrows anteriorly, convergent side walls, prominent ischial spines, and a narrow subpubic arch. Which Caldwell-Moloy type is this, and what is its principal obstetric implication?