When is pelvimetry indicated in pregnancy, which pelvic dimensions are measured, what threshold values define cephalopelvic disproportion, and how should the results guide the choice between vaginal delivery and cesarean section?

Pelvimetry in Pregnancy: Indications and Clinical Application

Current Evidence Does Not Support Routine Pelvimetry

Pelvimetry should not be routinely performed in pregnancy, as X-ray pelvimetry increases cesarean section rates without improving perinatal outcomes, and there is insufficient evidence to support the use of CT or MRI pelvimetry for predicting successful vaginal delivery. 1

Limited Indications for Pelvimetry

When Pelvimetry May Be Considered

• Skeletal dysplasia: Women with skeletal dysplasia characterized by short trunk or altered pelvic anatomy should be identified early, as cesarean delivery is recommended because pelvic anatomy often precludes vaginal delivery. 2, 3, 4

• Suspected cephalopelvic disproportion (CPD) with prolonged labor: When CPD cannot be ruled out clinically during labor arrest, imaging may help confirm the diagnosis, though clinical assessment remains primary. 3, 4

• Breech presentation at term: Pelvimetry combined with fetal measurements may help select candidates for vaginal breech delivery, though this remains investigational. 5

Clinical Diagnosis Remains the Standard

• Serial suprapubic palpation of the base of the fetal skull is the most critical examination technique to differentiate true descent from molding alone when evaluating for CPD. 3, 4

• CPD occurs in 25-30% of cases with active phase disorders and 40-50% of active phase arrest cases. 3, 4

Pelvic Dimensions Measured

Key Measurements in Radiological Pelvimetry

• Pelvic inlet: Anteroposterior diameter and transverse diameter (inlet circumference is a composite measure). 6, 7

• Midpelvis: Interspinous diameter (ISD) and bispinal diameter are critical as the midpelvis is often the narrowest point. 8, 6, 7

• Pelvic outlet: Intertuberous distance (ITD) between ischial tuberosities. 8

Fetal Measurements for Comparison

• Biparietal diameter (BPD) and head circumference (HC) are measured via ultrasound to assess fetal head size relative to maternal pelvis. 8, 6, 7

• Abdominal circumference (AC) provides additional information about overall fetal size. 8

Threshold Values for Cephalopelvic Disproportion

Evidence-Based Cutoffs

• Fetal head circumference ≥34.8 cm has 88% sensitivity and 74% specificity for predicting CPD requiring cesarean section. 3, 4

• Cephalopelvic disproportion index (CID) <9 mm: Vaginal delivery is impossible when the difference between the smallest pelvic diameter and fetal biparietal diameter is less than 9 mm (specificity 100%). 7

• CID 9-12 mm: Vaginal delivery is impossible or very difficult. 7

• CID ≥13 mm: Does not guarantee successful vaginal delivery (sensitivity only 51%), as other factors beyond bony dimensions affect labor progress. 7

Important Caveat on Threshold Values

• The relationship between pelvic dimensions and labor outcome is linear and relative to fetal size, not based on fixed maternal pelvic thresholds alone. 6

• Maternal inlet circumference (OR 0.95% CI 0.92-0.97), fetal HC (OR 1.05,95% CI 1.02-1.09), and maternal age (OR 1.09,95% CI 1.02-1.17) are independent risk factors for cesarean section in multivariable modeling. 6

Management Algorithm Based on Pelvimetry Results

When CPD is Confirmed or Cannot Be Ruled Out

• Proceed directly to cesarean delivery as the safest and most prudent option, as the risks of maternal and fetal damage are too great to attempt vaginal delivery. 3, 4

• Oxytocin augmentation is absolutely contraindicated when CPD is present, as it risks uterine hyperstimulation, uterine rupture, severe maternal lacerations, and fetal asphyxia without achieving vaginal delivery. 3, 4

• Operative vaginal delivery should not be attempted when CPD is confirmed, as this represents a clear contraindication. 4

When CPD is Ruled Out

• If CPD is definitively excluded through complete cefalopelvimetric evaluation, vaginal delivery is preferable, provided it does not prove difficult. 3

• A trial of labor may continue with appropriate monitoring. 3

Special Populations Requiring Cesarean Regardless of Measurements

• Women with skeletal dysplasia should have cesarean delivery planned early in pregnancy with multidisciplinary coordination, including anesthesia evaluation for airway and neuraxial anatomy. 2, 9

• Management of delivery should be discussed early in pregnancy, including location, mode of delivery, and anesthesia management to optimize maternal and fetal outcomes. 2

Critical Clinical Pitfalls

Avoid These Common Errors

• Do not use artificial rupture of membranes for arrest of dilation, as there is no objective evidence this is useful treatment. 3

• Do not continue oxytocin or instruct pushing when CPD cannot be excluded, as this is both futile and dangerous. 3

• Do not rely on pelvimetry alone: Clinical assessment combined with fetal measurements provides better prediction than pelvic dimensions alone. 6, 5

Risk Factors Requiring Heightened Suspicion for CPD

• Maternal factors: Diabetes, obesity, skeletal dysplasia. 3, 4

• Fetal factors: Macrosomia, malposition, malpresentation, marked asynclitism, excessive molding without descent. 4

Quality of Evidence and Limitations

• The Cochrane review found low to very low-quality evidence for X-ray pelvimetry, with increased cesarean rates (RR 1.34,95% CI 1.19-1.52) but no clear differences in perinatal mortality or morbidity. 1

• Advanced 3D/4D sonography and other imaging techniques to assess spatial relationships between fetal skull and maternal pelvis remain under investigation, as current diagnostic methods for CPD remain imperfect. 4

• MRI pelvimetry shows promise in research settings (AUC 0.799 for predicting vaginal delivery), but lacks sufficient evidence for routine clinical use. 8