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).
The placental stage of labour ends with:
The production of prostaglandins in the endometrium: