Preterm labor is among the most common and most serious of prenatal complications. Preterm labor and its potential sequelae of preterm delivery and low-birth-weight (LBW) infants remain one of the most significant challenges of current obstetrical practice. Preterm labor is defined as uterine contractions resulting in progressive cervical change prior to 37 weeks’ gestation. Preterm delivery is delivery between 20 and 37 weeks of gestation. LBW infants are defined as those infants weighing less than 2500 g at delivery regardless of gestational age. LBW infants should be distinguished from small-for-gestational-age (SGA) infants who are defined as those infants below the tenth percentile for weight based on gestational age.