How do you diagnose and rule out cephalopelvic disproportion (CPD) in a nulliparous woman at 37 weeks gestation with an unfavorable cervical exam and lack of response to misoprostol (prostaglandin E1 analogue)?

Cephalopelvic Disproportion (CPD): Diagnosis and Management

Understanding CPD in Your Clinical Scenario

In a nulliparous woman at 37 weeks with an unfavorable cervix unresponsive to misoprostol, CPD cannot be definitively diagnosed until active labor with adequate contractions, but cesarean delivery should be strongly considered if labor induction fails or if clinical signs of CPD emerge during labor. 1, 2

What is CPD?

CPD represents a mechanical mismatch between fetal head size and maternal pelvic dimensions that prevents safe vaginal delivery. 3 This occurs in 40-50% of all active phase arrest cases, making it the most critical factor to evaluate when labor fails to progress. 1, 2

Critical Limitation in Your Case

CPD cannot be reliably diagnosed before active labor begins. 3 Your patient at 37 weeks with cervical ripening failure has not yet entered active labor, so true CPD assessment is premature. However, failed cervical ripening may signal underlying mechanical obstruction. 1

Clinical Signs That Rule IN CPD (During Active Labor)

When labor is established, look for these specific findings:

• Increasingly marked molding of the fetal skull without descent - this is the hallmark sign 1, 2
• Deflexion and asynclitism of the fetal head without descent despite contractions 1
• Serial suprapubic palpation showing no descent of the base of the fetal skull - this is the single most critical examination technique to differentiate true descent from molding alone 1, 2
• Arrest of cervical dilation after reaching ≥5 cm, unresponsive to oxytocin augmentation, after active dilatation of ≥2 cm in 2 hours 4

Risk Factors That Increase CPD Likelihood

Assess these factors in your patient:

• Maternal factors: diabetes, obesity (higher BMI increases CPD risk) 2, 5, 6
• Fetal factors: macrosomia (weight >3.5 kg), fetal head circumference ≥34.8 cm (88% sensitivity, 74% specificity for CPD) 2, 5, 7
• Positional factors: malposition (occiput posterior/transverse), malpresentation (brow presentation) 1, 2

How to Rule OUT CPD

CPD is ruled out when:

• Normal labor progression occurs with adequate cervical dilation (≥1 cm/hour in active phase) and fetal descent 3
• Suprapubic palpation demonstrates progressive descent of the fetal skull base (not just molding) 1, 2
• No excessive molding, deflexion, or asynclitism develops 1

Management Algorithm for Your Specific Case

Step 1: Current Assessment (37 weeks, failed misoprostol)

• Document maternal BMI, estimated fetal weight, and any ultrasound measurements of fetal head circumference 2, 7
• Perform clinical pelvimetry to assess pelvic adequacy 3
• If multiple risk factors present (obesity, diabetes, estimated fetal weight >3.5 kg), counsel regarding increased cesarean risk 2, 6

Step 2: If Proceeding with Labor Induction

• Consider alternative ripening agents or mechanical methods 1
• Oxytocin is absolutely contraindicated if CPD is suspected or confirmed - it risks uterine rupture, severe maternal lacerations, excessive fetal molding, and asphyxia without achieving vaginal delivery 1, 2, 8
• Monitor closely for signs of CPD once active labor begins 1

Step 3: During Active Labor

• Perform serial suprapubic palpation of the fetal skull base every 1-2 hours to assess true descent 1, 2
• Document molding, asynclitism, and position changes 1
• If arrest of dilation occurs with signs of CPD, proceed immediately to cesarean delivery 1, 2

Step 4: When CPD is Confirmed or Cannot Be Ruled Out

• Cesarean delivery is mandatory - the American College of Obstetricians and Gynecologists explicitly states "it is better to err on the side of intervention by cesarean delivery in the presence of uncertainty about potential harm than to allow labor in the false hope that safe vaginal delivery may occur" 1
• Do not attempt oxytocin augmentation 1, 2, 8
• Do not attempt operative vaginal delivery 2

Critical Pitfalls to Avoid

• Never use oxytocin when CPD is suspected - the FDA label explicitly contraindicates oxytocin in "significant cephalopelvic disproportion" 8
• Do not rely on molding alone as evidence of descent - use suprapubic palpation of the skull base 1, 2
• Do not delay cesarean delivery hoping for spontaneous resolution - risks of maternal trauma (uterine rupture, severe lacerations) and fetal trauma (excessive molding, asphyxia) are too great 1, 2
• Recognize that astute clinicians often identify CPD before formal arrest criteria are met, especially with malposition or excessive molding 1

Special Consideration for Future Pregnancies

If cesarean is performed for strictly defined CPD (arrest after ≥5 cm unresponsive to oxytocin, excluding malposition), 68% of women can achieve vaginal delivery in subsequent pregnancies, particularly if the next baby is smaller. 4 This should inform counseling about future delivery mode.