33. Physiology of labour for KEATS

Learning Outcomes

  • Describe the phases of parturition.

  • Explain the stages of labour and physiological processes in each stage, including hormonal changes.

  • Understand uterine contractions and cervical changes.

  • Recognize the role of fetal position and movements in labour progression.

Introduction

  • Labour expels fetus, placenta, and membranes after 24 weeks gestation.

  • Considered the third phase of parturition and is a continuum:

    • Onset of labour

    • First stage (latent and established)

    • Second stage (passive and active)

    • Third stage

  • Influenced by hormones, contractions, cervical changes, and fetal movements.

  • Understanding these processes aids midwifery support and identification of abnormalities.

Parturition Phases

  • Quiescence: Relaxed, inactive during most of pregnancy.

  • Activation: Preparation for labour, softening occurs in the last weeks.

  • Stimulation: From onset of labour to birth; characterised by contractions and expulsion.

  • Involution: Post-delivery phase where the uterus returns to its pre-pregnancy state.

Onset of Labour

  • Occurs ideally between 37 and 42 weeks.

  • Before 37 weeks is termed preterm birth.

  • Induction of labour is common at >42 weeks.

  • Fetal lung maturation occurs 4-6 weeks before birth.

Hormonal Changes in Labour

  • Increased oestrogen and decreased progesterone.

  • Release of oxytocin from maternal and fetal pituitary glands.

  • Prostaglandins released from cervix and uterus.

  • Fetus releases cortisol.

First Stage of Labour

  • Latent Phase: Initial contractions lead to cervical changes (0-4cm dilatation).

  • Established Phase: Regular contractions (+4cm dilation).

  • Average duration:

    • Primigravid: ~8h (max 18h)

    • Multiparous: ~5h (max 12h)

Uterine Contractions

  • Involuntary and occur 3-4 times every 10 minutes during the established phase.

  • Ferguson's reflex: fetal pressure on cervix prompts oxytocin release, enhancing contractions.

  • Uterine segments: Fundus contracts strongly, lower segment weakly aiding cervical dilatation.

Cervical Effacement and Dilation

  • Cervical effacement: Merging of cervix with lower uterine segment (LUS).

  • Cervical dilation must reach 10cm for delivery.

  • Mucus plug often dislodges as cervix effaces.

Second Stage of Labour

  • Begins once cervix is fully dilated; ends with the birth of the baby.

  • Passive and active phases defined by pushing efforts.

  • Duration of active phase:

    • Primigravid: up to 3h

    • Multiparous: up to 2h.

Third Stage of Labour

  • Begins with the birth of the baby, ends with the birth of the placenta.

  • Placental separation aided by uterine contractions and formation of retroplacental haematoma.

Control of Bleeding

  • Blood flow to the uterus is ~750ml/min at term.

  • Three mechanisms prevent postpartum hemorrhage:

    • Uterine retraction acts as living ligatures around blood vessels.

    • Contraction applies pressure to placental site.

    • Clot formation is supported by increased coagulation post-separation.

Oxytocin and Prostaglandins

  • Oxytocin: Key hormone in childbirth, influencing contractions and emotional response.

  • Prostaglandins: Involved in cervical ripening, uterine contractions, and postpartum involution.

Labour Progress and Biomechanics

  • Labour dystocia is linked to mechanical imbalances affecting fetal positioning.

  • Understanding biomechanics aids midwives in facilitating optimal positions during labor.