Cephalopelvic Disproportion (CPD)
The term for a mother whose pelvis is too small for vaginal delivery is cephalopelvic disproportion (CPD).
Definition and Clinical Context
Cephalopelvic disproportion refers to an anatomical misfit between the maternal pelvis and the fetal head, where the infant's cranium is too large to pass through the birth canal 1. This condition represents one of the most common obstetric complications and is a major indication for cesarean delivery, particularly in sub-Saharan Africa 2.
Diagnostic Criteria
CPD is diagnosed when cervical dilation arrests after 5 cm and remains unresponsive to oxytocin augmentation, following active dilatation of at least 2 cm over 2 hours 3. The diagnosis requires:
- Serial suprapubic palpation of the base of the fetal skull to differentiate true descent from molding, rather than relying solely on vaginal examination 4
- Thorough cephalopelvimetric assessment before considering any intervention 4
- Exclusion of fetal malpresentations and malpositions 3
Associated Risk Factors
CPD occurs in 25-30% of cases with active phase labor disorders 1. Key risk factors include:
- Maternal factors: diabetes, obesity 1
- Fetal factors: macrosomia 1
- Strong association with arrest of descent (approximately 52% of cases in nulliparous women) 4
Clinical Measurement Tools
The mid-pelvic cephalopelvic circumference index (MP-CPCI), representing the ratio between head circumference and pelvic circumference (HC/PC × 100), can distinguish between lower and higher risk of CPD 5. Each 1% increase in MP-CPCI increases the likelihood of cesarean delivery for CPD by 11% 5.
A head circumference of 34.8 cm or greater has 88% sensitivity and 74% specificity for predicting CPD requiring cesarean section 2.
Management
Cesarean delivery is the safest and most prudent option when there is evidence of CPD or when it cannot be ruled out with reasonable certainty 1. Key management principles include:
- Oxytocin is contraindicated when there is evidence of CPD, as it may cause uterine rupture and endanger both mother and fetus 1
- Complete cephalopelvimetric evaluation is vital before continuing with oxytocin or considering operative vaginal delivery 1
- When the cephalopelvic disproportion index (smallest pelvic diameter minus biparietal diameter) is less than 9 mm, vaginal delivery is impossible 6
Special Populations
In women with skeletal dysplasia, pelvic anatomy precludes vaginal delivery in most cases, and cesarean delivery is recommended 7. The infant's cranium will be too large to pass through the birth canal whether the infant has normal stature with normocephaly or short stature with macrocephaly 7.
Important Caveat
A strictly defined diagnosis of nulliparous CPD should not automatically constitute a "recurrent" indication for elective cesarean delivery in subsequent pregnancies, as 68% of such patients may achieve successful vaginal delivery in their next pregnancy 3. This rate is similar to outcomes reported after all nulliparous cesareans for dystocia 3.