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OG8.5 | Pelvic Assessment and CPD Evaluation — SDL Guide (Part 2)

Interpreting Pelvimetry Findings and Diagnosing CPD

Interpreting the pelvimetry findings requires converting the measured diagonal conjugate to the clinically important obstetric conjugate and then synthesising all findings to reach a classification of pelvic adequacy relative to the expected fetal presenting part size. This interpretation step is where the procedural skill of pelvimetry is translated into a clinical decision — and it is where examination candidates most commonly make errors, either by forgetting to subtract 1.5–2 cm when converting DC to OC, or by interpreting individual measurements in isolation without integrating the pelvic type (android pelvis with borderline measurements is more concerning than gynaecoid pelvis with the same measurements). The classification framework below provides a structured approach: it is not a set of rigid cut-offs but a guide that must be combined with gestational age, estimated fetal weight, engagement status, and the woman's obstetric history before a management plan is reached.

Provided image

Converting diagonal conjugate to obstetric conjugate:
The obstetric conjugate (OC) is the shortest anteroposterior diameter of the pelvic inlet — the minimum space available for the fetal head at the inlet. It cannot be measured directly because the sacral promontory is not accessible to a finger from the anterior direction. The clinical approximation is:
- Obstetric conjugate = Diagonal conjugate − 1.5 cm (if pubic symphysis height is normal ≈ 3.5–4 cm)
- Obstetric conjugate = Diagonal conjugate − 2 cm (if pubic symphysis is tall/deep, more common in women with android-type pelvis)
- In practice, subtracting 1.5–2 cm is used and interpreted in range: a DC of 12 cm → OC of 10–10.5 cm (borderline/normal); a DC of 11 cm → OC of 9–9.5 cm (contracted inlet)

Classification of CPD:
- No CPD (adequate pelvis): OC ≥10 cm, bispinous ≥10 cm, bi-ischial ≥8 cm, normal subpubic angle — vaginal delivery is anticipated to be possible with appropriate fetal position
- Borderline CPD: measurements within 1 cm of thresholds, or one borderline + normal others — trial of labour with close monitoring is appropriate
- Suspected CPD: OC 8–10 cm, or bispinous <10 cm with prominent ischial spines + android pelvis characteristics — consider trial of labour with senior oversight or elective LSCS depending on estimated fetal weight and obstetric history
- Absolute CPD: OC <8 cm — vaginal delivery of a term fetus is not possible; elective caesarean section is indicated

Additional clinical factors in CPD assessment:
- Estimated fetal weight (EFW): a smaller fetus (EFW 2.8 kg) may achieve vaginal delivery through a pelvis that would obstruct with a macrosomic fetus (EFW 4.2 kg)
- Moulding and asynclitism: the fetal skull bones can overlap at the sutures (moulding) by up to 1 cm, effectively reducing the presenting diameter — this is a physiological compensatory mechanism
- Engagement: if the fetal head is engaged at term in a primigravida (head has descended below the pelvic brim), the inlet measurements are functionally adequate
- Pelvic type: android pelvis is the most unfavourable; gynaecoid the most favourable, even with borderline measurements

Applied Decision-Making: Managing Suspected or Confirmed CPD

The management of CPD requires integrating pelvimetry findings with multiple clinical variables to arrive at the safest delivery plan for mother and fetus. No single pelvimetric measurement determines the delivery mode in isolation — the decision emerges from a combination of pelvic dimensions, estimated fetal weight, gestational age, parity, fetal presentation and position, and the strength and regularity of uterine contractions. This reflects the 'three P's' framework: the passage (pelvis) is assessed by pelvimetry, but the passenger (fetus) and the powers (contractions) must also be adequate for vaginal delivery.

Absolute CPD (OC <8 cm):
Elective lower segment caesarean section (LSCS) is indicated before the onset of labour. The risk of maternal and fetal harm from obstructed labour far outweighs the risks of planned caesarean delivery. The decision should be made by 36–38 weeks gestation in conjunction with a senior obstetrician.

Borderline or suspected CPD (OC 8–10 cm):
Trial of labour is the standard management — allowing the woman to enter spontaneous labour and monitoring progress closely. The trial of labour proceeds with the following conditions:
- Continuous CTG monitoring from the active phase
- Senior obstetrician review every 2 hours in active labour
- Expectation of normal progress (1 cm/hour after 4 cm in active phase)
- Threshold for abandoning the trial: arrested labour at any phase despite adequate uterine contractions; evidence of fetal compromise; developing caput succedaneum and moulding without descent
- Emergency LSCS if the trial of labour fails (documented as 'failed trial of labour' or 'failure to progress secondary to suspected CPD')

The trial of labour is contra-indicated in:
- Absolute CPD (OC <8 cm)
- Previous uterine rupture
- Classical (upper segment) caesarean scar (vertical uterine scar)
- Fetal malpresentation that is unfavourable for vaginal delivery (e.g., brow presentation, face presentation with mentoposterior)

Role of imaging in borderline CPD:
CT pelvimetry provides more precise measurements than clinical pelvimetry and is used in specialist centres for truly borderline cases. However, most obstetric decisions do not require CT — clinical judgement combined with ultrasound EFW is sufficient. X-ray pelvimetry is no longer recommended due to fetal radiation exposure.

Self-Assessment: Pelvic Assessment and CPD

Pelvimetry is a skill assessed in your DOAP station — you will be asked to demonstrate the technique on an obstetric pelvic model, interpret the measurements, and describe your management plan. The questions below consolidate both the technical and clinical reasoning elements of this competency, targeting the level of performance expected in an SH (Should Demonstrate Hands-on) competency assessment. An SH assessment means you must not only describe the technique verbally but also perform it correctly on the model — your hand position for the diagonal conjugate, the way you identify the sacral promontory, how you mark the measuring point on your hand, and how you state the result and calculate the obstetric conjugate. Reviewers in DOAP stations particularly watch for: correct patient/model positioning, systematic coverage of all three pelvic levels (inlet, midplane, outlet), and the ability to integrate findings into a management recommendation that is appropriate for the obstetric context. The questions below consolidate both the technical and clinical reasoning elements of this competency. Attempt each question as if you are at the DOAP station: describe the technique as you would perform it, state the measurements as you would record them, and give the management as you would advise a patient.

Self-check questions for structured review:
- Describe the step-by-step technique of measuring the diagonal conjugate. What structure do you need to reach with your examining finger, and how is the measurement taken from the hand?
- A diagonal conjugate of 10.5 cm is measured. What is the estimated obstetric conjugate? How would you classify this pelvis and what delivery plan would you recommend?
- Name the four Caldwell-Moloy pelvic types. Which is the most favourable for vaginal delivery, and which features make the android pelvis the least favourable?
- At what point in labour, and with what specific findings, would you decide that a trial of labour for borderline CPD should be abandoned?
- State the normal threshold for each of the following: (1) obstetric conjugate, (2) diagonal conjugate, (3) bispinous diameter, (4) bi-ischial diameter.