OG13.2-3 | Mechanism and Physiology of Normal Labour — Summary & Reflection

KEY TAKEAWAYS

Normal labour is defined by spontaneous onset, vertex presentation, term gestation, and normal progress. Labour onset is driven by the fetal HPA axis → oestrogen rise → prostaglandin synthesis (PGE2 for cervical ripening via collagenases; PGF2α for myometrial contractility) → oxytocin receptor upregulation. The Bishop score (5 parameters, maximum 13) quantifies cervical ripeness; ≥8 = favourable for induction. Three stages: first stage (onset to full dilatation; active phase ≥4 cm); second stage (full dilatation to delivery; up to 2 h primigravida/1 h multigravida); third stage (delivery of placenta; up to 30 min with AMTSL). Seven cardinal movements: engagement, descent, flexion, internal rotation, extension, external rotation, expulsion. Internal rotation is passive (pelvic floor guided); arrest produces deep transverse arrest. Partograph: alert line at 1 cm/h from 4 cm; action line 4 h to the right. AMTSL: oxytocin 10 IU IM with anterior shoulder delivery + CCT + uterine massage — reduces PPH by 60–70%.

REFLECT

Think back to any labour you have observed — in the skill lab on a mannequin, in simulation, or at the bedside. At what point in the second stage did you feel the head rotate internally? Could you feel the sagittal suture move? Now, knowing the seven cardinal movements and the pelvic planes that govern each step, replay that observation in your mind with this new framework. Which movement were you watching when the head 'crowned'? Which movement was the head completing when the midwife turned it outward after delivery? Kolb's model asks us to move from observation to reflection to conceptualisation — this exercise completes that loop and transforms a sequence of motions you saw into a physiology you now understand.