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Pelvimetry

Pelvimetry is the measurement of the dimensions and capacity of the pelvis to determine whether vaginal delivery is possible or if caesarean section may be required.

Overview

Definition: Assessment of pelvic dimensions to predict cephalopelvic disproportion (CPD) and feasibility of vaginal delivery
Clinical Significance: While routine pelvimetry is not recommended in modern obstetrics, understanding pelvic anatomy remains important for:
  • Assessing progress of labour
  • Managing dystocia
  • Planning mode of delivery in specific cases
  • Understanding mechanisms of labour

Pelvic Anatomy

Pelvic Types (Caldwell-Moloy Classification)

Gynaecoid Pelvis (50% of women)
  • Round inlet
  • Wide subpubic arch (>90°)
  • Non-prominent ischial spines
  • Most favourable for vaginal delivery
Android Pelvis (20% of women)
  • Heart-shaped inlet
  • Narrow subpubic arch (<90°)
  • Prominent ischial spines
  • Convergent sidewalls
  • Least favourable for vaginal delivery
Anthropoid Pelvis (25% of women)
  • Oval inlet (anteroposterior > transverse)
  • Narrow subpubic arch
  • Non-prominent ischial spines
  • Occipitoposterior positions common
Platypelloid Pelvis (5% of women)
  • Flat, wide inlet (transverse > anteroposterior)
  • Wide subpubic arch
  • Rare; transverse arrest common

Key Pelvic Measurements

Pelvic Inlet (Superior Pelvic Aperture)
  • Anteroposterior diameter:
    • True conjugate (anatomical): 11 cm (from sacral promontory to upper pubic symphysis)
    • Obstetric conjugate: 10.5 cm (shortest AP diameter)
    • Diagonal conjugate: 12.5 cm (clinically measurable on vaginal examination)
  • Transverse diameter: 13 cm (widest distance)
  • Oblique diameter: 12 cm
Pelvic Cavity (Mid-pelvis)
  • Interspinous diameter: 10.5 cm (distance between ischial spines)
  • Narrowest part of pelvis
Pelvic Outlet (Inferior Pelvic Aperture)
  • Anteroposterior diameter: 13 cm (from lower pubic symphysis to tip of sacrum)
  • Transverse diameter (bi-ischial/intertuberous): 11 cm
  • Subpubic angle: >90° (gynaecoid)

Methods of Pelvimetry

Clinical Pelvimetry

Vaginal Examination
  • Assess diagonal conjugate (if sacral promontory can be reached, <12 cm suggests contracted pelvis)
  • Evaluate ischial spines prominence
  • Measure subpubic angle
  • Assess sacral curvature and coccyx mobility
  • Palpate sidewalls (parallel vs convergent)
Advantages: No radiation, readily available
Limitations: Subjective, examiner-dependent, less accurate

Radiological Pelvimetry

X-ray Pelvimetry
  • Anteroposterior and lateral views
  • Accurate measurements possible
  • Rarely used due to radiation exposure
CT Pelvimetry
  • Low-dose protocol available
  • Accurate 3D measurements
  • Minimal radiation (<0.25 mGy)
MRI Pelvimetry
  • No radiation
  • Excellent soft tissue visualization
  • Expensive, limited availability

Clinical Applications

Indications for Pelvimetry (Limited)

Possible Indications:
  • Breech presentation at term (controversial)
  • Previous traumatic pelvic fracture
  • Severe pelvic deformity (e.g., rickets, osteomalacia)
  • Trial of labour after caesarean section (TOLAC) - selected cases
NOT Routinely Indicated:
  • Normal pregnancy with cephalic presentation
  • Previous caesarean section (routine)
  • Suspected macrosomia
  • Failure to progress in labour (clinical assessment preferred)

Cephalopelvic Disproportion (CPD)

Definition: Mismatch between fetal head size and maternal pelvis preventing descent and delivery
Causes:
  • Pelvic factors: Contracted pelvis, abnormal pelvic shape
  • Fetal factors: Macrosomia, large head, malpresentation, malposition
  • Combined: Normal pelvis + normal baby but poor fit
Diagnosis:
  • Clinical assessment during labour (preferred)
  • Failure of descent despite adequate contractions
  • Caput and moulding assessment
  • Trial of labour often most reliable diagnostic tool
Management:
  • Trial of labour in most cases
  • Caesarean section if no progress despite adequate contractions

Current Recommendations

Evidence-Based Practice

WHO and ACOG Guidelines:
  • Routine pelvimetry NOT recommended
  • Does not improve outcomes
  • Does not accurately predict CPD
  • May lead to unnecessary caesarean sections
  • Clinical assessment during labour is more reliable
Trial of Labour:
  • Best assessment of fetopelvic relationship
  • Allows for dynamic assessment
  • Most women with suspected CPD can deliver vaginally

When to Consider Imaging

Breech Presentation:
  • Some centres use CT/MRI pelvimetry
  • Evidence limited
  • Clinical judgment remains important
Pelvic Trauma/Deformity:
  • Imaging may be helpful
  • Surgical consultation may be needed

Key Points

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  • Routine pelvimetry is not recommended in modern obstetrics
  • Clinical pelvimetry during labour provides dynamic assessment
  • Most CPD cannot be predicted before labour
  • Trial of labour is the best test of fetopelvic relationship
  • Imaging pelvimetry has limited specific indications
  • Understanding pelvic anatomy remains important for managing labour