Birthweight

The term "low birthweight" is used to describe infants who are born at the lower extreme of the birth-weight distribution. In 1948 the World Health Assembly recommended that a single definition of low birthweight (LBW) be established for consistent vital statistics and other public policy purposes. The current definition, a weight of less than 2,500 grams (approximately 5 pounds, 8 ounces), was derived from earlier recommendations by Ethel Dunham and Arvo Ylppo. Marked advances in medical technology and practice have occurred since the 2,500-gram criteria for LBW was established, resulting in vastly improved survival rates for LBW infants. The improvements in survival led to the need for further classifications of LBW to better identify high risk infants. Very small infants are now further categorized as very low birthweight (VLBW; less than 1,500 grams (3 pounds, 5 ounces)) and extremely low birthweight (ELBW; less than 1,000 grams (2 pounds, 3 ounces)).

The increased risk of poor outcome for LBW is illustrated by Figure 1. Of the single live births to U.S. resident mothers from 1995 to 1997, 6.1 percent were LBW and 1.1 percent were VLBW. Low birthweight and VLBW infants, however, made up 60 percent and 45 percent, respectively, of the infant deaths. The infant mortality rate for LBW infants was 63 deaths per 1,000 live-born LBW infants and was 259 deaths per 1,000 for VLBW infants.

Low birthweight includes both preterm delivery and fetal growth restriction, but these two categories have very different determinants. Despite extensive FIGURE 1 SOURCE: Martha Slay, Greg R. Alexander, and Mary Ann Pass. research, current knowledge is limited about the causes of preterm delivery. Risk factors associated with preterm birth include cigarette smoking during pregnancy, prior preterm birth, low prepregnancy weight, and maternal chronic diseases; but known risk factors account for less than one-fourth of preterm births. The factors associated with fetal growth restriction are more readily understood than those of preterm delivery. Cigarette smoking during pregnancy, low maternal weight gain, and low prepregnancy weight account for nearly two-thirds of all fetal growth restriction and seem to be the most promising areas for possible interventions. Other associated factors include multiple births (e.g., twins), infant gender, and several factors relating to the mother, including: birthweight, racial or ethnic origin, age, height, infections, history of prior low birthweight delivery, work/physical activity, substance use/abuse, cigarette smoking, alcohol consumption, and socioeconomic status. While prenatal care was once touted as a highly effective means to prevent low birthweight, more recent assessments have raised serious challenges to this assumption, leaving the matter now in doubt.

Poverty, given its association with reduced access to health care, poor nutrition, lower education, and inadequate housing, may be an appreciable factor underlying the risk of delivering a LBW infant. Socioeconomic status is linked to individual behaviors, such as cigarette smoking and alcohol consumption, and varies markedly by race and ethnicity. While socioeconomic status and race/ethnicity cannot be termed "causes" of low birthweight, they serve as indicators of complex links between environmental, psychological, and physiological factors that may result in higher risks of low birthweight.

The percentage of LBW infants in the United States rose during the last two decades of the twentieth century. This increase, coupled with the improved survival of LBW infants, has heightened the need to further understand the long-term outcomes of LBW infants in regard to growth, development, and disease, as well as the impact these children have on the health care system. When compared to normal birth-weight children, LBW children have higher rates of mental retardation, cerebral palsy, blindness, deafness, psychomotor problems, school failure, subnormal growth, and health problems, which are compounded by poverty and related adverse socioeconomic factors.