Physiology of labor

Physiology of laborBy; Tiyas KTheory of laborProgesterone deprivationOxytocinUterine muscle stretchProstaglandinPlacental agingFetal cortisol

Progesterone deprivation

oxytocin

Uterine stretch

Fetal cortisol

Stage of laborFirst stageSecond stageThird stageFour/ observational stageFirst stage of laborLatent phaseFew days leading up to active labourProstaglandin mediated ripening of cervixIrregular contractions begin effacementBloody show mucous and blood which previously plugged cervix liquefies. Likely also mediated by prostaglandins.Membranes can rupture at any time (often assisted but ARM is NOT part of normal labour). This event tends to trigger active labour, again likely due to the release of prostaglandins (true mechanism not fully understood)

Active phaseActive phase Said to begin once regular contractions established, or effaced cervix 3cm dilatedCervix dilates at approx 1cm/hr and is incorporated into lower segmentUpper segment progressively shortens and thickens, due to spirals of smooth muscle contractingLower segment stretches and thinsEnds when cervix is 10cm dilated (Fully)

Second stage of laborBegins when cervix fully dilatedLasts about 1hr in primip, 30mins in multip but lenthened by epidural analgesiaUpper segment continues to shorten and thicken. Majority of fetus in lower segmentHead passes intraspinous diameter the narrowest part of the pelvisPerineum softened by congestion with blood (not unlike arousal)Delivery accomplished by the following six manoeuvres:

Second stage of laborEngagement. Descent and flexionInternal rotation. Delivery by extensionExternal rotationExpulsion Third stage of laborFrom delivery of fetus to delivery of placentaUsually within 15-30 mins (depends on choice of active or expectant management)Immediately after delivery, contractions tend to stop for brief periodPlacenta separated due to shearing effect of uterus contracting after foetus delivered, thereby reducing size of site of attachmentRetroplacental haematoma forms, exuding downwards pressureActive management is now so common to be considered NORMALOxytotic (commonly syntocinon) given by IM injection to stimulate uterine contractionPlacenta can be delivered by maternal effort or by controlled cord traction (CCT)Active Management has been shown to reduce PPH

Fourth stage of laborObservation of:ContractionHemorrhage Fundal height EliminationVital signs

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