Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005
Low Birth Weight in the United States
Low birth weight is a widely used and much studied marker of infant health.1 It is well measured, reliably recorded, and readily available from vital statistics files and many other data sets. Birth weight is often categorized as very low (less than 1,500 grams, or about 3.3 pounds), low (less than 2,500 grams), or normal (2,500 grams or more). Further distinctions include extremely low (less than 1,000 grams) and moderately low (1,500–2,499 grams) birth weight. Births can also be characterized by gestational age: very preterm (less than 32 weeks), preterm (less than 37 weeks), and term (37 weeks or more). These terms and their definitions are summarized in table 1, along with the corresponding rates of births in the United States in 2000. Babies considered small for gestational age (SGA) or growth retarded are typically below the 10th percentile in sex-specific birth weight for gestational age. All low birth weight babies are preterm or growth retarded (they can be both), and virtually all very low birth weight babies are preterm.
Babies born in the United States are more likely to be low birth weight than those born in almost every other developed country.2 Low birth weight is the second leading cause of infant mortality in the United States after birth defects, and surviving infants are at elevated risk for debilitating medical conditions and learning disorders.3 Figure 1 shows rates of low birth weight, very low birth weight, and infant mortality (death before age one) in the United States from 1980 to 2000. Thanks to increased specialization in delivering maternal and newborn health care and to advances in neonatal intensive care technology, the United States made substantial progress in reducing the infant mortality rate over this period, although its gains have lagged behind those of other developed countries.4 Rates of low and very low birth weight, meanwhile, increased slightly, owing partly to the increasing prevalence of multiple births; the rate of low birth weight among singleton births has remained steady, at about 6 percent.5
Low birth weight babies are much more likely to survive today than they once were. Since 1960, survival rates have increased dramatically for very low and extremely low birth weight babies born in the United States (figure 2). Although less than 10 percent of extremely low birth weight singleton infants born in 1960 lived to their first birthday, that figure increased to 27 percent for those born in 1980 and to 57 percent for those born in 2000.6 And while fewer than half of very low birth weight (defined here as 1,000–1,499 grams) singleton babies born in 1960 survived, by 2000 the share surviving had increased to more than 90 percent. Likewise, the survival rates of moderately low birth weight singleton infants increased from 91 percent in 1980 to 98 percent in 2000.7 The new survivors, however, are at high risk for health and developmental problems, as discussed below.
Survival and Disability
The majority of moderately low birth weight infants thrive, suffering few or no problems. It is the lightest babies who are most at risk of disabilities, both cognitive and physical, that can impair school readiness. Of the many child health conditions associated with low birth weight, perhaps the most potentially disabling is cerebral palsy, a group of disorders characterized by the inability to control movement and often accompanied by cognitive impairments.8 Preterm very low birth weight infants are up to 30 percent more likely to develop cerebral palsy than are babies born at term.9 Other serious conditions associated with low birth weight or preterm birth include mental retardation, respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), retinopathy of prematurity (ROP), and deafness. RDS and BPD can lead to feeding difficulty, recurrent respiratory infections, asthma, and growth delay.10 ROP, a disorder caused by abnormal growth of blood vessels in the eye, can lead to blindness.11 All these disabilities can impair learning and inhibit a child's school readiness. Almost without exception, the prevalence of these disabling conditions increases as birth weight decreases.
A recent review of forty-two studies of infants born after 1970 found no change between 1976 and 1990 in the prevalence of major neurodevelopmental disabilities among extremely immature (26 weeks or less) and extremely small (800 grams or less) survivors. Throughout that period, cerebral palsy affected 12 percent of extremely immature and 8 percent of extremely small survivors; mental retardation affected 14 percent of each group; 8 percent of each group was blind; and 3 percent of each group was deaf. Overall, 22 percent of extremely immature survivors and 24 percent of extremely small survivors had at least one major disability.12