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Progesterone to prevent preterm birth in twin gestations: what is the next step forward?

Progesterone is a key hormone for pregnancy maintenance, and a decline in progesterone action is considered to be central to the initiation of parturition in most mammalian species, including primates.1, 2 This hormone acts in all components of the common pathway of parturition1-3 and promotes myometrial quiescence, inhibits cervical ripening, and down-regulates the production of chemokines [such as interleukin (IL)-8] by the chorioamniotic membranes.4 Importantly, the administration of progesterone receptor antagonists [i.e. mifepristone (RU486) or onapristone (ZK98299)] to pregnant women, non-human primates and guinea pigs can induce the onset of labor and cervical ripening.5 Thus, it is not surprising that many investigators have explored the use of progesterone for the prevention of preterm birth.