stages of labour

parturition:
Means birth of the baby, toward the end of the pregnancy the uterus increase its excitability due to hormonal and mechanical factors.
Then strong uterine contraction develop ending by expulsion of the fetus.
1- hormonal factors: in the mother increase estrogen to progesterone ratio, and increased secretion of the oxytocin by the posterior pituitary.
in the fetus increased oxytocin, incr. cortisol, incr. prostaglandin from the fetal membranes.
2- mechanical factors: stretching of the smooth muscle lead to incr. contractility, so contractions can be induced by fetal movements and over distended uterus in twin pregnancy.
The baby is delivered and this process is called labour, and the contractions are called labour contractions.
Labor contractions continue to occur by a +ve feedback mechanism when the uterine cervix is stretched by the fetal head this will lead to contractions of the uterine body and increase the oxytocin secretion by the post. Pit.
Fergusson reflex
As labour becomes established, the output of oxytocin increases through the Fergusson reflex.
Pressure from the fetal presenting part against the cervix is relayed via a reflex arc involving the spinal cord and results in increased oxytocin release from the maternal posterior pituitary