Figure 7. Conceptual diagram of changes occurring in the uterus during pregnancy and labour.

Early in pregnancy the uterus becomes quiescent with a decrease in responsiveness to contractile agonists. This is accompanied by a decrease in agonist receptor expression and increased expression of relaxatory mechanisms. During pregnancy, there is gradually increasing postreceptor coupling of agonist receptors to the mechanisms that increase contractility leading to increasing spontaneous contractile activity and greater maximal response to OT. At the time of uterine activation, there is increased production of uterine agonists (such as OT and ET-1) from the endometrium, accompanied by increased expression of agonist receptors in the myometrium and decreased expression of relaxatory systems. At this time, there also is increased expression of other genes (e.g. Cx-43) that facilitate the coordination and efficiency of uterine contractions that are characteristic of active labour.