During pregnancy progesterone levels are high and maintain inhibitory actions on uterine muscle contractions. When the end of pregnancy is approaching, estrogen, oxytocin and prostaglandin levels increase. Estrogen is produced by the placenta and increases the sensitivity of uterine smooth muscle to oxytocin. Higher oxytocin production in the pituitary is evoked by these estrogens and by distortion of the uterus (due to the growing foetus). Oxytocin increases the force and the frequency of uterine muscle contractions.
Oxytocin and estrogens stimulate a higher production of prostaglandins in the uterus. The prostaglandins further stimulate contractions. The actual trigger for the onset of labour might come from the foetus itself. The foetal pituitary gland secretes oxytocin, which is released in the maternal bloodstream. The resulting increase in myometrial contractions and PG production on top of estrogen and maternal oxytocin is the beginning of the delivery process.
The placental stage of labour ends with:
The production of prostaglandins in the endometrium: