Obstetric Topic 26. Cephalopelvic disproportion and its consequences

I. Definition and Etiology

Definition

• Cephalopelvic disproportion (CPD): fetal head/body is too large to pass through the maternal pelvis.
• Fetopelvic disproportion: broader term including fetal size, malposition and pelvic capacity.
• Core clinical problem: adequate uterine contractions but failure of cervical dilation and/or fetal descent.
• Often diagnosed during labor, not perfectly predicted before labor.

Etiology

1. Maternal / pelvic causes: contracted inlet/midpelvis/outlet, short stature, congenital pelvic abnormality, pelvic fracture, rickets/osteomalacia, kyphosis/scoliosis.
2. Obstructing maternal mass: fibroid, ovarian cyst/tumor, pelvic tumor, cervical/vaginal stenosis.
3. Fetal causes: macrosomia/diabetic fetopathy, hydrocephalus, malposition (OP/OT, asynclitism), malpresentation (brow, face, shoulder).

Contracted Pelvis

• Contracted inlet: classic criteria AP diameter <10 cm or transverse diameter <12 cm.
• Contracted midpelvis: common cause of transverse arrest and difficult instrumental delivery.
• Contracted outlet: may contribute to dystocia and severe perineal tears, often with midpelvic contraction.
• Small maternal height (<150 cm) increases suspicion but does not diagnose CPD.

II. Diagnosis

Clinical Signs

• Failure of fetal head engagement near term or during labor.
• No descent despite strong/adequate contractions.
• Arrest of cervical dilation or second-stage arrest.
• Excessive caput succedaneum and molding; may falsely suggest descent.
• Early rupture of membranes, abnormal presentation, cord prolapse risk in contracted inlet.
• Bandl's ring/pathologic retraction ring → impending obstructed labor/uterine rupture.

Investigations

• Vaginal examination: dilation, station, caput/molding, position, pelvic adequacy.
• Leopold maneuvers: engagement and fetal position.
• Ultrasound: fetal weight, biparietal diameter/head circumference, presentation/position, hydrocephalus.
• Clinical pelvimetry: inlet, midpelvis, outlet assessment; supportive, not absolute.
• CT/MRI pelvimetry: rarely used; selected complex pelvic deformity or previous trauma.

III. Consequences of Unmanaged CPD

Maternal

• Prolonged/obstructed labor → exhaustion, dehydration, ketosis.
• Uterine rupture, especially with strong contractions against obstruction.
• Postpartum hemorrhage: uterine atony, trauma or rupture.
• Puerperal infection/sepsis; later infertility from severe pelvic infection.
• Pressure necrosis → vesicovaginal, vesicocervical or rectovaginal fistula.
• Pelvic floor injury and chronic pelvic morbidity.

Fetal / Neonatal

• Fetal distress, hypoxia, birth asphyxia.
• Birth trauma: skull fracture, cephalohematoma, nerve injury, intracranial hemorrhage.
• Infection after prolonged rupture/labor.
• Stillbirth or neonatal death if obstruction is untreated.

IV. Management

Principles

1. Assess 3 P's: power, passenger, passage.
2. Confirm contractions are adequate; use oxytocin only if contractions are inadequate and CPD is not severe.
3. Continuous fetal and maternal monitoring during trial of labor.
4. Minor/suspected CPD with reassuring FHR and progress → careful trial of vaginal labor.
5. Severe CPD, obstructed labor, fetal distress, uterine rupture risk or no progress despite adequate contractions → cesarean section.
6. Second-stage arrest: consider operative vaginal delivery only if head is low/engaged, position known, pelvis adequate and operator skilled; otherwise cesarean.