The exact cause of onset of labor is not known. Why do labor begins spontaneously at completion of term is an enigma.
Probably there is interactions of various factors that stimulates inert, relaxed, gravid uterus and make it responsive to contractile forces.
1- Uterine Distension.
2- Fetus and Membranes.
Before the onset of labor, for unknown reasons, the Anterior pituitary gland of fetus produces more Adreno-cortico trophic hormone that in turn stimulates the adrenal gland of fetus to produce more Cortisol hormone.
The cortisol hormone stimulates the fetal membranes to produce more Estrogen and Progesterone hormones.
The fetal membranes on stimulation by stretching or irritation by infection/ inflammation also produces Prostaglandins that play a vital role in onset of labor.
3- Oxytocin Hormone.
Before the onset of labor, the concentration of oxytocin receptors in uterine muscles increase several fold. This makes the muscles hypersensitive to the oxytocin hormone that is released from the posterior pituitary gland and leads to uterine contractions.
4- Neurological Factors.
Stimulation of Alpha receptors of Post Ganglionic Autonomic Fibres situated around the uterine cervix play a important role in onset of labor.
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