The pharmacologic inhibition of premature labor.

Oxytocin, elevated estrogen-progesterone ratio, fetal corticosteroids, prostaglandins, catecholamines, and changes in uterine blood flow have all been implicated as triggers of labor. In approximately one-third of cases of threatened premature labor contractions stop spontaneously. Thus placebo-controlled randomized trials of any new drug for inhibition of premature labor are necessary, as the spontaneous cessation of contractions always favors the claimed therapeutic efficacy. Alcohol inhibits the release of endogenous oxytocin and has an additional direct effect on the myometrium. In one study alcohol was more effective than placebo in the postponement of delivery. Isoxsuprine, ritodrine, and terbutaline have also been shown to be better than placebo in the inhibition of premature labor, and the beta adrenergic agents appear to be more effective than alcohol. Prostaglandin inhibitors such as indomethacin are currently under investigation. Success is correlated with early administration of the therapy, which requires treating some patients whose contractions might have stopped spontaneoulsy. As different factors may be involved in triggering premature labor, if one therapeutic approach fails another should be initiated promptly.