Causes for onset of labour

Theories regarding initiating of labour include the following -

1. Oxytocin -
There is oxytocin receptor in the uterus. Oxytocin receptors are increased
in the uterus with the onset of labour. And then oxytocin promotes the
release of prostaglandins from the decidua. Oxytocin synthesis is increased
in the decidua and in the placenta. Oxytocin level reaches the maximum at 
the moment of birth.

2. Progesterone -
A decrease in progesterone production may stimulate prostaglandins (PG)
synthesis and enhance the effect of estrogen which has a stimulating effect on
uterine muscle.

3. Estrogen -
The probable mechanisms are-
 - Increases the release of oxytocin from maternal pituitary.
 - Promotes the synthesis of receptors for oxytocin in the myometrium and decidua.

4. Prostaglandins -
Prostaglandins are the important factors which initiate and maintain labour. 
Prostaglandins stimulates smooth muscle to contract. The major sites of synthesis
of prostaglandins are amnion, chorion, decidual cells and myometrium.