Prematurity is the leading cause of perinatal morbidity and mortality in the industrial world, occurring in 4% to 9% of all deliveries, a rate that has remained unchanged during the past decades. Despite the relative minority of obstetric patients affected by this problem, prematurity is responsible for approximately 70% to 80% of perinatal morbidity and mortality corrected for congenital anomalies. To date, treatment modalities (tocolysis) that have been applied to patients who have preterm labor (PTL) and preterm premature rupture of membranes have been found to be of limited value in reducing both the rate of prematurity and of perinatal mortality and morbidity. A possible explanation for this failure in prevention of prematurity can be attributed to the poor understanding of the mechanisms of parturition in general and the pathophysiology of PTL in particular.