Medically indicated preterm birth: recognizing the importance of the problem.

Preterm birth complicates over 500,000 births annually, affecting 12.5% of pregnancies in the United States. Much of the temporal increase in preterm birth (<37 weeks) over the past decade is largely driven by a concurrent temporal increase in medically indicated preterm birth. Maternal and fetal indications that prompt an intervention at preterm gestational ages include preeclampsia, intrauterine growth restriction, and placental abruption-conditions that constitute "ischemic placental disease." Ischemic placental disease is implicated in over one of every two indicated preterm births compared with less than one in five births at term. Comprehensive evaluation of risk factors, with careful consideration of heterogeneity in the syndrome of medically indicated preterm birth and ischemic placental disease may provide important clues to predict and consequently prevent preterm birth.