Assessment of the Female Bony Pelvis
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Lesser pelvis is the bony canal through which the foetus has to pass during childbirth.
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It is therefore of great importance to determine the diameter of this canal .
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The diameter can be determined by a pelvic examination or radiographically.
Internal Pelvimetry
Pelvic Inlet
Diagonal conjugate:
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Try to palpate the sacral promontory to measure the diagonal conjugate.
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Normally, it is 12.5 cm and cannot be reached.
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If it is felt the pelvis is considered contracted and the true conjugate can be calculated by subtracting 1.5 cm from the diagonal conjugate
The mid-cavity
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Shape and inclination of the sacrum.
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Side walls: To determine whether it is straight, convergent or divergent •
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Ischial spines: Whether it is blunt (difficult to identify at all), prominent (easily felt but not large) or very prominent (large )
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By using the 2 examining fingers, if both spines can be touched simultaneously, the interspinous diameter is 9.5 cm i.e. inadequate for an average-sized baby.
Pelvic outlet
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Subpubic angle: Normally, it is 90 °
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Bituberous diameter: Normally, it admits the closed fist of the hand (4 knuckle).
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Mobility of the coccyx. by pressing firmly on it while an external hand on it can determine its mobility. Anteroposterior diameter of the outlet: from the tip of the sacrum to the inferior edge of the symphysis (>11.0 cm)
ROTATION OF FETAL HEAD
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Widest diameter of pelvic canal changes from transverse diameter at pelvic inlet to AP diameter at pelvic outlet
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To obtain best fit of fetal head, the longest diameter of the fetal head passes through the widest diameter of the pelvis. Therefore the head must rotate during labour
