Difficulties in controlling and preventing preterm labor in multiple gestations: a clinical perspective.

Prevention of preterm labor or arrest of ongoing premature labor in a twin gestation is more difficult to achieve than in singleton pregnancies. This clinical perspective is intended to review currently applied methods and to propose a more effective means to resolve this problem. A number of tocolytic methods have been employed in multifetal gestations, but none is routinely effective once true labor has commenced. Whereas a fall of serum progesterone levels precedes the onset of labor in animals, such is not the case in humans. It is proposed here that the level of serum progesterone does not define the relevance of this hormone to the maintenance of myometrial quiescence in humans. Rather, it would appear to be the progesterone supplied by the placenta directly to the myometrium that delays the onset of labor. The ratio of placental surface area to myometrial surface (P/M) as the gestation progresses more accurately reflects the capability to delay labor, especially in twin pregnancies. Hence, the application of therapeutic progesterone in closer proximity to the placenta/myometrial interface (e.g., via the vagina) should be more effective than by injection in preventing preterm labor as the P/M ratio decreases in a multifetal gestation.