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The clinical onset of labour, characterised by painful uterine contractions leading to cervical dilatation, is a relatively late event in a series of biochemical changes that occur throughout the pregnancy, but particularly in the late third trimester. For most of pregnancy, myometrial contractile activity needs to be repressed, whilst the cervix needs to remain firm and closed to retain the developing fetus within the uterus. Nearer to term, the cervix ripens, a process characterised by dissociation of collagen and a decrease in its concentration and an increase in water content (Osmers et al. 1995). This results in a looser matrix so that cervical tissue offers lower resistance to force and its collagen fibres will deform under tension.
In the myometrium, with the onset of labour, fundally dominant contractions begin. Cervical tissue is drawn up into the lower segment of the uterus, in the processes of effacement and dilatation. Eventually with the establishment of labour, the lower segment itself is drawn up towards the upper segment and the fetus is pushed through the birth canal and into the outside world. These changes within the uterus are associated with changes in the expression of a range of genes that Steven Lye in Toronto has termed ‘contraction-associated proteins’ (CAPs). In the myometrium such proteins include those that form gap junctions (Sparey et al. 1999), proteins in the prostaglandins synthetic pathways (Slater et al. 1999) and receptors for oxytocin (Fuchs et al. 1984) and prostaglandins (Erkinheimo et al. 2000).
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