CAUSES OF ONSET OF LABOR

In a woman when pregnancy reach at term and why a pregnant woman feels labor pain and how contraction started and delivery occur
When fetus matures at term of pregnancy fetal hypothalamus get activated and Release CRH( corticotropin releasing hormone)
This corticotropin releasing hormone stimulates fetal pitutary gland .
Due to stimulation of CRH fetal pitutary gland secreate ACTH( Adrenocorticotropic hormone)
This ACTH stimulate fetal adrenal glands.
Due to stimulation of ACTH fetal adrenal gland secreate cortisol and dwhydroepiandrosterone sulphate.( DHEAS)
DHEAs go to the placenta and converted to oestrogen.
Cortisol go to placenta and placenta secreate estrogen and Prostaglandin.
Prostaglandin secret from decidua, fetal membrane, amnion and chorion and from placenta.
Here estrogen secreation increase and progesterone secreation decrease.
Due to this estrogen and progesterone ratio alter .
This rise in estrogen level stimulates maternal pitutary gland to release oxytocin.
High level of estrogen increase oxytocin receptor in myometrium
High level of estrogen cause secretion of Prostaglandin.
Due to oxytocin and Prostaglandin labor pain starts due to uterine contraction.

These are all steps why labour pain starts in pregnancy.