OG8.5 | Pelvic Assessment and CPD Evaluation — Summary & Reflection

KEY TAKEAWAYS

Cephalopelvic disproportion (CPD) is the mismatch between the fetal presenting part and the maternal pelvis. Clinical pelvimetry assesses three levels: inlet (diagonal conjugate, measured clinically; obstetric conjugate = DC minus 1.5–2 cm, normal ≥10 cm), midplane (bispinous diameter, assessed by ischial spine prominence, normal ≥10 cm), and outlet (bi-ischial diameter ≥8 cm, subpubic angle >90°). The Caldwell-Moloy classification identifies four pelvic types: gynaecoid (round, most favourable), android (heart-shaped, least favourable, deep transverse arrest risk), anthropoid (oval AP, OP delivery likely), platypelloid (flat, difficult engagement). Technique of diagonal conjugate measurement: insert two fingers, reach sacral promontory with middle finger, mark where examining hand meets inferior pubic symphysis border, measure = DC. Bi-ischial diameter measured with closed fist (≥8 cm = adequate). Contracted pelvis: OC <10 cm. Absolute CPD (OC <8 cm) = elective LSCS. Borderline CPD (OC 8–10 cm) = trial of labour with senior monitoring. Engagement of fetal head at term in primigravida = functional evidence of adequate inlet.

REFLECT

Return to Sumathi's case from the opening hook. She was 142 cm tall with no ANC pelvic assessment. Now work through what should have happened: (1) At her 36-week ANC visit, what risk factors were present that should have prompted formal pelvimetry? (2) If her diagonal conjugate had been 9.5 cm, what would the obstetric conjugate be, and how would you have classified her pelvis? (3) With a contracted inlet, what would your management plan have been, and at what gestational age would you have planned the caesarean section? (4) Sumathi developed a vesicovaginal fistula from obstructed labour — what obstetric mechanism caused this injury? Working through this case using the pelvimetry framework demonstrates how a single competent clinical assessment at 36 weeks can change the entire delivery plan and prevent a catastrophic maternal outcome.