Clinical Pelvimetry and Trial of Labour
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Direct answer
Clinical pelvimetry is a bedside assessment of the inlet, midpelvis and outlet, estimating whether the bony birth canal can accommodate the fetal head. Because it predicts cephalopelvic disproportion poorly, the decision to deliver vaginally is made through a monitored trial of labour rather than by examination alone. A trial is reasonable when the fetus is cephalic, the pelvis is not grossly contracted and labour progresses on the partograph.
What you must remember
- Diagonal conjugate: measured from the sacral promontory to the lower border of the symphysis, usually about 12 cm, and below about 11.5 cm suggests a contracted inlet.
- Obstetric conjugate: about 1.5 to 2 cm less than the diagonal conjugate, and below about 10 cm suggests a contracted inlet.
- Interspinous diameter: around 10 cm is adequate, and a narrow midpelvis raises the risk of arrest in the second stage.
- Subpubic angle: a narrow arch limits the capacity of the outlet.
- Caldwell-Moloy shapes: gynaecoid is the most favourable, anthropoid is acceptable, while android and platypelloid are unfavourable.
- Radiological pelvimetry: abandoned because of radiation exposure, with MRI or ultrasound as alternatives if needed.
- Partograph: the alert and action lines define failure to progress, and caesarean is considered when the action line is crossed.
- Predictive value: clinical pelvimetry alone is a weak predictor of outcome, so labour behaviour is decisive.
Assessing the inlet, midpelvis and outlet
Pelvic assessment follows the three planes in sequence, with the shape of the pelvis judged last. Findings from each plane are recorded separately, because a narrow midpelvis matters even when the inlet looks adequate.
- Inlet: Estimate the diagonal conjugate by vaginal examination, then derive the obstetric conjugate by subtracting about 1.5 to 2 cm. Examination of the sacral promontory is limited in practice, so the estimate is approximate.
- Midpelvis: Palpate the ischial spines and sacral curve, and judge whether the interspinous diameter is adequate. Prominent spines and a flat sacral curve narrow the midpelvis, and this plane is often implicated in deep transverse arrest.
- Outlet: Judge the subpubic angle, the sacrococcygeal space and the outlet diameters, noting any narrow arch. Outlet narrowing is often the problem when a low head fails to deliver.
- Pelvic shape: Classify the pelvis as gynaecoid, anthropoid, android or platypelloid, since the shape predicts the likely direction of descent.
- Clinical integration: Combine the findings with estimated fetal size, presentation and position to judge whether a trial of labour is suitable. Ultrasound estimation of fetal weight helps when clinical estimation is uncertain.
Decision traps on the pelvis
Pelvimetry questions test the limits of the bedside examination and the logic of the trial.
- Trap: Ordering a caesarean on pelvimetry alone Correct: clinical pelvimetry predicts disproportion poorly, so a monitored trial of labour is the usual decision.
- Trap: Using X-ray or CT pelvimetry routinely Correct: radiation exposure makes it unacceptable, and MRI or ultrasound are the alternatives.
- Trap: Continuing a trial after the partograph action line is crossed Correct: failure to progress past the action line calls for caesarean delivery.
- Trap: Treating an android pelvis as an absolute bar to vaginal birth Correct: an android shape predicts a difficult descent and deep transverse arrest, but the outcome depends on progress.
Frequently asked questions
What is the obstetric conjugate and what length is needed?
The obstetric conjugate is the shortest antero-posterior diameter of the pelvic inlet, about 1.5 to 2 cm less than the diagonal conjugate. A value below about 10 cm suggests a contracted inlet and a higher risk of cephalopelvic disproportion.
Which pelvic shape is most favourable for vaginal delivery?
The gynaecoid pelvis is the most favourable shape in the Caldwell-Moloy classification, with the anthropoid pelvis also acceptable. Android and platypelloid pelvises are less favourable, so the trial of labour and partograph are followed more closely.
When is a trial of labour appropriate in a suspected contracted pelvis?
A trial of labour is appropriate when the pelvis is only borderline, the fetus is cephalic, labour is progressing on the partograph and a caesarean can be performed promptly. Maternal and fetal condition must allow close monitoring throughout.
What do the alert and action lines on a partogram signify?
The alert line marks the point where cervical dilatation is slower than expected, and the action line lies four hours to the right of it. Crossing the action line without improvement prompts consideration of caesarean delivery.