Birth (Parturition)
The onset of birth is signaled by the reduction in progesterone level in the mother’s blood, which allows the maternal posterior pituitary gland to release hormone oxytocin.
Oxytocin removes the inhibitory effect on contractions of the myometrium whose muscles start contracting.
The release of oxytocin occurs in “waves” during “labour” and provides force to expel the foetus from the uterus.
The onset of contractions of myometrium, called “labour pains” are accompanied by the dilation of the cervix, rupture of the amnion and chorion releasing the amniotic fluid through the cervix, and the stimulation of stretch receptors in the walls of the uterus and cervix.
The contraction of the uterus wall follows.
Uterine contraction spreads down over the uterus thus pushing the foetus downwards, head first through the widened cervix and birth canal (vagina) and the baby is born.
After birth, the umbilical cord is ligatured and cut in order to separate the baby from the placenta. The placenta is expelled later as after-birth.
After birth and expulsion of the placenta, important changes occur in the baby’s circulation associated with the fact that respiratory function of the placenta is taken over by the lungs. The newborn takes the first breath and the lungs expand and become functional.