Preterm birth (delivery prior to 37 weeks or 259 days of gestation) complicates 1 in 8 deliveries in the United States, but accounts for over 85 percent of all perinatal morbidity and mortality. Efforts to delay delivery in women presenting with acute preterm labor have been largely unsuccessful. For this reason, much attention has focused on preventative strategies.
●(See "Cervical insufficiency".)
●(See "Second trimester evaluation of cervical length for prediction of spontaneous preterm birth".)
ROLE OF PROGESTERONE IN PREGNANCY MAINTENANCE
Progesterone is a steroid hormone initially produced by the corpus luteum. It is critical for the maintenance of early pregnancy until the placenta takes over this function at 7 to 9 weeks of gestation, and its name is derived from this function: pro-gestational steroidal ketone. Indeed, removal of the source of progesterone (the corpus luteum)  or administration of a progesterone receptor antagonist  readily induces abortion before 7 weeks (49 days) of gestation.
The role of progesterone later in pregnancy, however, is less clear. Progesterone appears be important in maintaining uterine quiescence in the latter half of pregnancy, possibly by limiting the production of stimulatory prostaglandins and inhibiting the expression of contraction-associated protein genes within the myometrium, including ion channels, oxytocin and prostaglandin receptors, and gap junctions [3-5]. Although levels of progesterone in the maternal circulation do not change significantly in the weeks preceding labor, the onset of labor both at term and preterm is associated with a functional withdrawal of progesterone activity at the level of the uterus [3-10].