What is a Clinically Contracted Pelvis?
A clinically contracted pelvis refers to a pelvic structure with dimensions inadequate to allow safe vaginal delivery of the fetus, diagnosed through clinical assessment of pelvic shape, size, and capacity in relation to fetal size—a condition that signals cephalopelvic disproportion (CPD) and necessitates cesarean delivery to prevent maternal and fetal injury. 1
Clinical Definition and Recognition
A contracted pelvis represents a mechanical obstruction to labor where the bony pelvis cannot accommodate passage of the fetal head, regardless of adequate uterine contractions. 1 This diagnosis requires assessment of multiple factors beyond simple pelvic measurements:
- Pelvic shape and size abnormalities that restrict the birth canal 1
- Fetal macrosomia in relation to pelvic capacity 1
- Fetal malposition (occiput posterior and transverse) or malpresentation (brow) 1
- Asynclitism and excess molding of the fetal skull 1
Critical Diagnostic Considerations
The diagnosis is fundamentally clinical rather than purely radiographic. 1 Key assessment points include:
- Serial suprapubic palpation of the base of the fetal skull to differentiate between molding and true descent—this is essential to confirm actual fetal descent is occurring 1
- Active-phase protraction or arrest disorders that suggest CPD 1
- Maternal factors including diabetes and obesity that increase risk 1
When CPD Cannot Be Ruled Out
If evidence of CPD is found with an active-phase protraction or arrest disorder, or if it cannot be ruled out with reasonable certainty, cesarean delivery is the more prudent and safer choice. 1 The risks of maternal and fetal damage are too significant to attempt vaginal delivery when CPD is suspected, as safe vaginal delivery becomes unlikely or unachievable. 1
Historical Context and Modern Understanding
Older obstetric literature defined contracted pelvis based on specific pelvic inlet measurements (typically area <100 cm²), with "relative contracted pelvis" for borderline cases (100-110 cm²). 2 However, modern practice recognizes that:
- Three-dimensional pelvic assessment is more clinically relevant than single measurements 2
- Sacral shape and deformity significantly impact delivery outcomes beyond inlet dimensions 3, 2
- Previous normal deliveries do not guarantee future vaginal delivery, as pelvic changes (such as osteomalacia) can occur 4
Associated Labor Abnormalities
A prolonged deceleration phase combined with failure of descent is a harbinger of second-stage abnormalities and frequently accompanies contracted pelvis. 1 This combination makes safe vaginal delivery very unlikely and increases risk of:
Management Imperative
Uterotonic stimulation is inadvisable when contracted pelvis is suspected, as it risks unsafe uterine hyperstimulation without addressing the mechanical obstruction. 1 The priority must be recognition of the anatomical limitation and proceeding to cesarean delivery to optimize maternal and fetal outcomes.