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Childbirth (presentation of the fetus)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The normal presentation is cephalic - i.e. head down - and occiput anterior - i.e. face downwards. It is this presentation that gives the best fit, presenting the smallest diameter to the birth canal.

In normal (flexed) presentation the presenting diameter is suboccipito-bregmatic, measuring on average 9.5 cm.

The different presenting antroposterior diameters of the fetus are presented below:

  • attitude = flexed; presentation = vertex; mean presenting diameter = suboccipito-bregmatic (9.5 cm)
  • attitude = deflexed +; presentation = deflexed; mean presenting diameter = occipito-frontal (11.5 cm)
  • attitude = deflexed ++; presentation = brow; mean presenting diameter = mento-vertical (13.5 cm)
  • attitude = deflexed +++; presentation = face; mean presenting diameter = submento-bregmatic (9.5 cm)

The biparietal diameter (average 9.5 cm) will be unaffected.

The different dimensions of the bony passages through which the fetus must pass through for a vaginal delivery are, on average:

  • pelvic bone
    • inlet: AP = 11.5 cm; transverse = 13.5 cm
    • mid-cavity: AP = 12.5 cm; transverse = 12.5 cm
    • outlet: AP = 13.5 cm; transverse = 11.5 cm

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