Birthweight is an important indicator of the approximate maturity of a newborn infant and the ability of that newborn infant to survive. The birthweight of an infant is dependent on the duration of the pregnancy and its rate of fetal growth. Infants who are delivered earlier than normal are expected to be of smaller birthweight than average. Additionally, infants who had slower or faster fetal growth can also have lower or higher than usual birthweights. Figure 1 portrays the birthweight distribution of singleton live births (babies born singly) to U.S. resident mothers from 1995 to 1997. The graph reveals a somewhat bell-shaped distribution with most births (about 80%) concentrated between 2,750 and 4,250 grams (between 6 pounds and 9 pounds, 4 ounces). The median birth-weight for U.S. singleton, full-term (forty weeks of gestation) births is nearly 3,500 grams (7 pounds, 11 ounces).
The close relationship between an infant's birth-weight and the risk of dying within the first year of life has long been recognized, and birthweight is often used by researchers as a measure of mortality risk. At light and heavy birthweights, an infant's risk of mortality soars (see Figure 1), although in recent decades, heavier infant births have become less associated with high mortality risks, probably because of medical intervention. Nevertheless, very light infants continue to be at grave risk of mortality, morbidity (disease), and long-term developmental problems.
Populations with more infants born at very high or very low birthweights predictably have higher infant mortality rates. Therefore, it is an established procedure to take birthweight into account when making comparisons of mortality among newborn populations. Whether the comparison involves temporal, geographic, socioeconomic, hospital, or other contrasts, infant mortality differences are typically examined within birthweight categories. Investigations of improving trends in infant mortality rates often start with an examination of the extent to which any changes are related to improvements in the distribution of birthweights within categories (e.g., fewer births at extreme birthweights), as opposed to reductions in birthweight-specific mortality rates (e.g., infants in specific birthweight categories having better survival).
Nearly all of the decline in infant mortality rates in the United States in the last quarter of the twentieth century was due to improvements in survival rather than any improvement in the birthweight distribution. Better survival within birthweight groups has been attributed to advances in obstetric and newborn medical care. The increasing medical care costs that have accompanied these advances, however, raise concerns about overly relying on medical technology to reduce infant death rates. Accordingly, research attention has been directed at finding the determinants of low birthweight in order to develop more cost-effective, population-wide programs to further diminish infant mortality.
Variations of average birthweight have been associated with infant gender, multiple birth factors, and maternal factors, such as race and ethnicity, size, nutrition, and current and previous pregnancy medical risk characteristics. One of the unresolved questions among researchers is whether there is a single common average human birthweight or whether there are normal variations in average birthweight among population subgroups. This question entails important medical care, public health policy, and political aspects as it engenders debate about what is a "normal" birthweight, what is a "high-risk" birthweight, and whether a single "one-size-fits-all" criteria for high-risk birthweights is equally valid for all infants.
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 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.
High birthweight (HBW), or macrosomia (large body), in an infant also increases the risk to the infant and mother. A widely agreed upon definition of macrosomia has yet to be established but often-used definitions include a birthweight equal to or exceeding 4,000 grams (8 pounds, 12 ounces), 4,250 grams (9 pounds, 4 ounces), or 4,500 grams (9 pounds, 14 ounces), as well as a birth weighing at or above the ninetieth percentile of birthweights for the infant's gestational age. While one-third of macrosomic births are still unexplained, several factors are known to contribute to excessive fetal size, including large size of parents (especially the mother), multiparity diabetes in the mother, and prolonged gestation. Older maternal age, male infants, and previous delivery of a high birthweight infant also seem to be indicative of macrosomic births. Babies of diabetic women are usually large at birth, but they behave clinically as if they are immature. These infants are not longer in average length but have increased fetal weight. Because glucose, a substance necessary for fetal growth, is elevated in both diabetic and obese women, these mothers are more likely to have macrosomic births.
Risks for birth injuries rise rapidly for heavier babies, with vaginal deliveries being related to higher morbidity and mortality for both the infant and the mother. Lacerations of the birth canal and hemorrhaging may occur to the mother, fetal death may occur due to asphyxia (lack of oxygen), and infants may suffer broken clavicles and neurological damage. While cesarean delivery has been prescribed as the best delivery method to prevent fetal death or injury, others suggest that vaginal birth is still possible for some macrosomic infants.
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