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The baby engages in the widest diameter of the pelvic inlet = Transverse (13cm)
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Enters the circular cavity (12cm) and rotates
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Exits via the widest diameter the antero-posterior (AP) of the pelvic outlet (13cm)
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The birth canal is formed by the bony pelvis and soft tissues
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To negotiate the birth canal, the fetus must take advantage of the widest diameter of each part of the pelvis
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The baby needs to twist and turn as it makes its way through the birth canal and adapt to the shape of the mothers pelvis
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The pelvic inlet is widest in the transverse diameter and the pelvic outlet is widest in the anteroposterior
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Lie is longitudinal
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Attitude is fully flexed
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Presentation is cephalic
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Position is right or left occipitoanterior
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Presenting part or Denominator is the occiput
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The fetus descends
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The presenting part of the fetus meets resistance of the pelvic floor
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The presenting part rotates forwards to lie anteriorly under the symphysis pubis
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The emerging part of the fetus pivots around the pubic bone
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Descent occurs due to: uterine contractions, amniotic fluid pressure, abdominal muscle contractions
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Descent is encouraged by: Increased abdominal muscle tone, Braxton hicks, fundal based uterine contractions, increased frequency and strength of contractions
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- Descent
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- Flexion
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- Internal rotation of the head
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- Crowning and extension of the head
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- Restitution
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- Internal rotation of the shoulders and external rotation of the head
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- Lateral flexion
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As the head descends, it moves towards the pelvic brim in either the left or right occipitotransverse position
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The baby descends through the pelvic inlet towards the pelvic floor
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As the fetus descends through the pelvis, it changes position to accommodate the widest part of the woman's pelvis
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Contractions push down on the spine causing the fetal head to come in contact with the pelvic floor which leads to flexion of the fetal head.
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The increased flexion leads to the presentation of the smallest diameter of the fetal head (sub-occipito bregmatic), which assists passage through the pelvis.
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Internal rotation of the head aligns the head with the anteroposterior diameter of the pelvic outlet and moves the occiput forwards to lie under the symphysis pubis
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This is caused by pressure from uterine contractions, the shape of the pelvic floor, the pressure of the ischial spines
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It causes a slight twisting of the neck and now the head is no longer in line with the shoulders
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The occiput now moves from beneath the pubic arch and pushes against the vaginal entrance
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The head is born by extension and pivots on the suboccipital region around the pubic bone
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The sinciput, face and chin sweep the perineum
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Restitution occurs after the birth of the head
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The head returns to its original position in correct alignment with the shoulders
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This occurs when the shoulders turn to fit the widest diameter of the pelvic outlet (anteroposterior)
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The anterior shoulder reaches the pelvic floor and rotates forwards to lie under the symphysis pubis
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This causes external rotation of the head so the occiput lies laterally towards the woman's thigh
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The anterior shoulder is usually born first under the pubic arch
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The spine undergoes lateral flexion to accommodate the curved birth canal and the rest of the body is born in that position
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