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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:
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:
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