Physiology of parturition

Abstract

A normal course of parturition requires both softening and dilatation of the cervix. Cervical softening already begins during late gestation under the influence of relaxin and/or oestrogens and occurs more rapidly close to term when the progesterone dominance is waning and uterine prostaglandin (PG) production increases. Biochemical changes within the ground substance of the connective tissue fibres result in dilution or breakdown of cervical collagen. Contrary to the active dilatation, this passive preparation of the soft tissues of the birth canal takes place in the absence of labour-like uterine contractions. Evidence in ruminants even suggests that the typical pattern of myometrial contractures, which can be recorded during the greater part of gestation, disappears during progesterone withdrawal, resulting in a prolonged period of complete myometrial quiescence, before labour-like contractions stan to develop. This may create a period of transformation during which several structural and functional changes take place within the uterine tissues, such as increased output of PGs by the placenta and fetal membranes, water uptake and collagenolysis in the cervix, synthesis of proteins involved in gap-junctions formation, and an increase of oxytocin receptors in the uterine wall. Maternal controlling mechanisms, such as an escape from direct myometrial inhibition by relaxin and catecholamines, or abolition of central inhibition of oxytocin release by relaxin and opioids, may finally switch on the myometrium to effect cervical dilatation and expulsion of the conceptus.