Causes of onset of labour

 Causes of onset of labour

1. Uterine distension

Stretching effect on myometrium due to growing of fetus and amniotic fluid.

Uterine stretch; increases gap junction protein, and receptors for oxytocin and specific contraction associated protein (CAPs)

2. Fetoplacental contribution

Active fetal hypothalamic pituitary adrenal axis; prior onset of labour.

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Increase CRH (corticotrophin- releasing hormone) 

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Stimulate anterior AP, increase release of ACTH (adrenocorticotropic hormone)

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Action fetal adrenals

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Increase cortisol secretion (for maturation of organ); accelerate the production of oestrogen and prostaglandins from placenta.

3. Oestrogen

Increases release of oxytocin from maternal pituitary.

Estrogen increase amount of oxytocin by activating cell that make oxytocin.

Increase number of oxytocin receptors

Promotes synthesis of myometrial receptors for oxytocin, prostaglandins and increase in gap junction in myometrial cell during parturition.

Lysomal disintegration;  Lysosomal enzymes are involved in connective tissue degradation.

Lysomal disintegration in decidua and amnion cell. Breakdown cellular component results increased prostaglandin synthesis.

Stimulate synthesis of myometrial contractile protein.

Increase excitability of myometrial cell membrane

4. Progesterone

Progesterone level fall before labour.

Increased fetal production of dehydroepiandrosterone sulphate (DHEA-S) and cortisol, inhibit production of progesterone.

5. Prostaglandins

Initiate and maintain labour.

Major sites of synthesis of prostaglandins are;

Amnion, Chorion, Decidual cells, Myometrium

Prostaglandins triggered by; 

Rise in oestrogen levels

Vaginal examination

Separation or rupture of membrane

Rise in oxytocin receptors (large number present in fundus compare to lower segment and cervix)

Prostaglandin enhance gap function formation.

Stimulate prostaglandin from amnion and decidua.

6. Nerve pathway

Labor may initiated through nerve pathways.

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Receptor present in Myometrium

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Contractile response initiated through receptors of nerve fibres in and around cervix and lower part of uterus.