The variety of factors controlling the onset of parturition, as well as the very complex mechanisms which physiologically maintain pregnancy by inhibiting uterine contractions are only partially understood today. As long as the triggering mechanisms, and the conditions favoring timely and early labor are incompletely known, symptomatic therapeutic attempts aim at blocking unwanted contractile activity of the uterine smooth muscle fibres. The high rates of neonatal mortality and morbidity associated with premature births [52] have stimulated efforts to find agents which safely inhibit uterine contractions in humans.