The Diagnosis and Management of Preterm Labor with Intact Membranes 1 . 3

introduction A preterm birth is one that occurs between fetal viability and 37 completed weeks of gestation (1–4). Delivery of a previable conceptus represents a spontaneous abortion rather than a preterm birth. Although the definition of “viability” varies among countries and even medical centers, the central idea is that a nonviable infant is so immature that there is no likelihood of survival in the extrauterine environment despite all medical support. Prior to modern developments in neonatal care, a nonviable infant was defined as one weighing less than 500g (see below). This definition is discouraged because preterm delivery should be defined by gestational age at birth, rather than birth weight. Although some have proposed to define a preterm birth as one that occurs between 20 and 37 weeks of gestation, we prefer to define preterm birth as one occurring between 24 and 37 weeks of gestation. Some neonates can survive if born around 24 weeks of gestation, but none at 20 weeks (5–8). This definition may need to be revised in the future if technological advances allow substantial survival of neonates born at less than 24 weeks of gestation. A birthweight of 500g has historically been used to define viability. However, this approach has limitations since viable neonates born after 24 weeks of gestation may be affected by intrauterine growth restriction (IUGR) and have birthweights below 500g. Conversely, some previable infants may weigh more than 500g. The threshold of 500g is valuable when there is uncertainty about gestational age. An accurate definition of preterm birth has implications for the calculation of vital statistics and comparisons of the rates of preterm delivery among different countries and populations, an issue that is often overlooked (1).