Answer
Pregnancy lasts about an average of 40 weeks (37-42 weeks). At the end of gestation contractions are initiated in the myometrium of the uterus which will eventually lead to the expulsion (birth) of the baby and the delivery of the afterbirth (placenta and residues of the extraembryonic membranes).
The first weak contractions of a near full-term or full-term uterus are sometimes called "false pregnancy" ( Braxton Hills contractions). These are facilitated by an increase in the level of estrogen which begins to over-ride the calming influence of progesterone. Associated increase of fetal lung surfactant also cooperates in producing a softening of the cervix. These effects are induced by the stimulation of the placenta by fetal cortisol.
In the next phase of labor fetal cells secrete oxytocin which leads to the secretion of prostaglandins from the placenta. These hormones cause further cervix softening and widening, and induce
stronger, expulsive, contractions of the myometrium. Both of these hormones are important in inducing the contractions of true labor, but prostaglandins are absolutely essential for normal labor to progress.
As the cervix widens ad stretches the hypothalamus is stimulated and the hypothalamic-posterior pituitary axis kicks in to pour out oxytocin into the blood stream. Strong positive feed-backs keep the oxytocin flowing and the myometrium contracting stronger and stronger until the baby is expelled. Of course the mother helps as she is timulated both neurlogically and byb hormone action to bear don and assist the autonomic contractions.
The whole procee of birth on the average takes eight to twelve hours. The delivery of the afterbirth--placenta and umbilcal cord andremnants of the extraembryonic membranes takes about 12 miutes
Work Step by Step
Two hormones are are most important in the initiation of the early contractios of labor. these are oxytocin and prostaglandin(s).
Estrogen secretion continues through pregnancy, but this is balanced by progesterone secretion which stabilizes the myometrium and prevents menstruation. However, near the end of pregnancy fetal cortisol stimulation of the placenta causes estrogen levels to rise . The effects of this enhanced level of estrogen is to increase the secretion of surfactant from the fetal lungs. this increases lubrication and softening of the cervix. The the myometrium is also induced to synthesise more oxytocin receptors, as well as create more gap junctions in the smooth muscle cells.
The increased estrogen level opposes the calming action of progesterone on the myometrium ; the smooth muscle cells become more sensitive to stimuli and begin weak contractions (Braxton Hills) sometime identified as "false labor".
Later in the process of labor, oxytocin secreted by the fetus causes the placenta to release prostaglandins. Both prostaglandin and oxytocin are myometrial stimulants, but prostaglandins are the stronger smooth muscle stimulant. The effect of these two hormones is to trigger strong expulsive contractions of true labor. Prostaglandin action also combines with the surfactant effect and changes in uterine fibronectin to soften widen of the cervix.
Th widening and stretching of the cervix sends powerful stimulatory messages to the hypothalamus which combines with the posterior pituitary to pour out more oxytocin into the blood stream
The activated hypothalamus responds with strong positive feedback reactions -- more and more oxytocin is released, and the uterine contractions get stronger and stronger, until the baby is delivered.
