Main hormone acting upon uterus to initiate labour
Oxytocin is a nonapeptide,
 It is secreted by posterior pituitary along with vasopressin (ADH).
Both oxytocin and ADH are synthesized within the nerve cell bodies in supraoptic and paraventricular nuclei of the hypothalamus; are transported down the axon and stored in the nerve endings within the neurohypophysis.
They are stored in separate neurones as complexes with their specific binding proteins (Neurophysin).

 Actions 

Uterus It increases the force and frequency of uterine contraction. Estrogens sensitize the uterus to oxytocin, while progestins decrease the sensitivity. The increased contractility is restricted to the fundus and body„ lower segment is not contracted, may even be relaxed at term.
Breast Oxytocin contracts myoepithelium of mammary alveoli and forces milk into bigger sinusoids - Milk ejection or milk letdown reflex.
CVS Higher doses cause vasodilation → Brief fall in BP, reflex tachycardia and flushing. The umbilical vessels are constricted oxytocin may help their clossure at birth.
Kidney Oxytocin in high doses exerts ADH like action —» water retention can occur.
Oxytocin
Estrogen
Progesterone
Cortisole