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