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Station 4 Dr J Herold Station learning objectives: 1. Understand what is meant by the term ‘mechanisms of labour’ 2. be able to describe the passage of the foetus through the birth canal in a skull presentation 3. understand what is meant by the following terms: - engagement - descent & flexion - internal rotation - extension - restitution - external rotation 4. demonstrate with a clear commentary the mechanisms of labour The passage (negotiating the ‘S’ bends) of the foetus through the pelvis / birth canal during the second stage of labour is known as the mechanisms of labour. 1. ENGAGEMENT The foetus is said to have engaged when the biparietal diameter has passed through the pelvic inlet, in an occipito-transverse position Engagement of the presenting part is assessed by: abdominal examination – 5ths above pelvic brim Leopold’s 4th manouevre deep pelvic grip - if your hands converge, presenting part is not engaged - if your hands diverge, presenting part is engaged completely above sinciput sinciput high, easily felt, occiput occiput easily felt felt sinciput felt, occiput just felt sinciput felt, occiput not felt none of head palpable 2. DESCENT Occurs as a result of the active forces (uterine contractions & bearing down) throughout labour, and the foetal head descends from the inlet into the cavity / mid-pelvis 3. FLEXION Occurs as a result of the resistance from the lower segment of the uterus and the levator ani muscles. Foetal head flexes onto chest 4. INTERNAL ROTATION Lowest presenting part (occiput in normal labour) encounters pelvic floor resistance and thus rotates to become anterior 5. EXTENSION Occurs in order for the foetal head to negotiate the natural curves of the pelvis therefore as extension occurs the parts of the foetal head move over the perineum in the following order: vertex, forehead, face & chin 6. RESTITUTION The head rotates (outside the body) the opposite of internal rotation in order to realign with the foetal shoulders which lie in the AP diameter behind the symphisis pubis 7. EXPULSION The anterior shoulder is delivered first from below the symphisis pubis, followed by the posterior shoulder over the perineum. Finally the trunk is born by lateral flexion. Following expulsion the umbilical cord is clamped.