Thus, neonates born before term can be small or large for gestational age, but still preterm by deinition. Low birthweight refers to neonates weighing 1500 to 2500 g; vey low birthweight are those between 1000 and 1500 g; and extremey low birthweight refers to those between 500 and 1000 g.
Before the 15th edition of this textbook, apreterm orpremature newborn was deined by a birthweight <2500 g. With that edition, preterm neonates were considered to be those delivered before 37 completed weeks, that is, <366r weeks (Pritchard, 1976). This definition, which has now been in use for more than 40 years, was first promulgated in 1976 by the World Health Organization (WHO) and the International Federation
FIGURE 42-1 Percentage of preterm births in the United States according to method of assessment of gestational age. LMP = date of last normal menses. (Adapted with permission from Martin JA, Osterman MJ, Kirmeyer SE, et al: Measuring gestational age in vital statistics data: transitioning to the obstetric estimate. Natl Vital
TABLE 42-1 . Infant Mortality Rates in the United States in 201e3
No. (%) (per 1000 births)
Total infants 3,932,1n81 (1n00) 23,446 (6)
Gestational age: <34 weeks 133,503 (3) 13,284 (100) 34-36 weeks 313,858 (8) 2268 (7) <37 weeks 447,361 (1n1) 15,552 (35) 37-38 weeks 974,162 (25) before 39 weeks (Spong, 2011). An unintended consequence of this health-care strategy has been a rise in stillbirth rates in the United States. One concern is that the rule may be misapplied to gestations with true medical indications for early delivery (Hill, 2017; Nicholson, 2016). Spong (2016) has emphasized the need to perform necessary obstetrical interventions when indicated.
39-41 weeks 2,291,468 (58) 4218 (2) ::42 weeks 215,510 (5) 515 (2)
Data from Matthews, 2015.
of Gynecology and Obstetrics (FIGO). The definition derived from a statistical analysis of gestational age distribution at birth (Steer, 2005). Importantly, the denotation lacks a specific functional basis and should be clearly distinguished from the concept ofprematuriy. Prematurity represents incomplete development of various organ systems at birth. For example, the lungs are particularly afected, leading to the respiratory distress syndrome (RDS) (Chap. 34, p. 636).
In 2013 in the United States, 23,446 infants died in their first year oflife, and a third ofinfants died from preterm-related causes (Matthews, 2015). Gestational age at deliveryand the risk of neonatal morbidity and mortality are inversely related (Frey, 2016). Namely, neonates born in the early-preterm period make up the smallest proportion of births, but these infants experience disproportionately higher rates of prematurity-related complications (Table 42-1).