Low birthweight defines neonates who are born too small. Preterm or premature births are terms used to describe neonates who are born too early. With respect to gestational age, a newborn may be preterm, term, or postterm. With respect to size, a newborn may be normally grown and appropriate for gestational age; undersized, thus, small for gestational age; or overgrown and consequently, large for gestational age. In recent years, the term small for gestational age has been widely used to categorize newborns whose birthweight is usually < 10th percentile for gestational age. Other frequently used terms have included fetal-growth restriction or intrauterine growth restriction. The term large for gestational age has been widely used to categorize newborns whose birthweight is > 90th percentile for gestational age. The term appropriate for gestational age designates newborns whose weight is between the 10th and 90th percentiles.
Thus, infants born before term can be small or large for gestational age but still fit the definition of preterm. Low birthweight refers to neonates weighing 1500 to 2500 g; very low birthweight refers to those between 500 and 1500 g; and extremely low birthweight refers to those between 500 and 1000 g. In 1960, a neonate weighing 1000 g had a 95-percent risk of death. Today, a neonate with the same birthweight has a 95-percent chance of surviving (Ingelfinger, 2007). This remarkable improvement in survival is due to the widespread application of neonatal intensive care in the early 1970s.
Up until the 15th edition of this textbook (1976), a preterm or premature infant was defined by birthweight < 2500 g. Beginning with the 15th edition, preterm infants were those delivered before 37 completed weeks, that is, ≤ 366/7 weeks. This definition, which has now been in use for almost 40 years, was first promulgated in 1976 by the World Health Organization (WHO) and the International Federation of Gynecology and Obstetrics (FIGO). This definition was based on a statistical analysis of gestational age distribution at birth (Steer, 2005). It lacks a specific functional basis and should be clearly distinguished from the concept of prematurity. Prematurity represents incomplete development of various organ systems at birth. The lungs are particularly affected, leading to the respiratory distress syndrome.
Beginning in 2005, in recognition that infants born between 340/7 weeks and 366/7 weeks experience morbidities and mortality characteristic of premature infants, preterm births were subdivided. Those before 336/7 weeks are labeled—early preterm, and those occurring between 34 and 36 completed weeks—late preterm. Most recently, Spong (2013) observed, “it has become apparent that infants born between 37 weeks 0 days and 38 weeks 6 days gestation experience morbidities that are associated with prematurity compared to births at 39 weeks 0 days through 40 weeks 6 days when infant mortality is lower than at any other time in human gestation.” Those births 370/7 weeks through 386/7 weeks are now defined as early term and those 39 weeks 0 days through 40 weeks 6 days are defined as term.