Epidemiology of late preterm (near-term) births.

The preterm birth rate (births before 37 completed weeks of gestation) has been increasing in the United States, largely driven by an increase in infants delivered between 34 and 36 weeks, often called near-term, but referred to as late preterm in this article. In 2004, the preterm birth rate was 12.5%, the highest rate since the National Center for Health Statistics began tracking such data. This article reviews the epidemiology of late preterm births and proposes a research agenda.