Epidemiology: the changing face of preterm birth.

Preterm birth, defined as a pregnancy ending at less than 37 completed weeks of gestation, is the leading cause of infant mortality in the United States. The occurrence of preterm births rose steadily from 9.4% of all pregnancies in the United States in 1981 to 12.8% in 2006, before declining to 12.7% in 2007 and 12.3% in 2008. Most of the increase was attributable to increases in multiple gestations. Recent research has sought to understand this condition by evaluating its familial occurrence and both clinical and pathologic information to derive an etiologically homogeneous categorization.