Programmed escalation: Models for the control of parturition

The biochemical and physiologic processes controlling labor in the human ha ve become much clearer in the past decade. Research has shed light on signa l mediators governing ripening of the cervix, activation of the uterus, and stimulation of the myometrium. Sex steroids, oxytocin, prostaglandins, cor ticotropin-releasing hormone (CRH) and cytokines have been studied intensiv ely. The fetoplacental unit contains positive feed-back loops triggering th e birth process in an escalating and irreversible manner. inflammatory-like reactions occur in a controlled and selective manner during parturition. N ew models have been developed to explain the timing of birth. Bioavailable maternal CRH levels increase strongly near term, suggesting that CRH is an indicator of a placental clock determining the duration of pregnancy. In an other model, the birth process is initiated by placental inactivation of ma ternal cortisol, thus inducing maturation of the fetal hypothalamic-pituita ry-adrenocortical axis. How other signal mediators are integrated and how t he various processes are coordinated remains unclear.