Low Birthweight

Low Birthweight
The term ‘‘low birthweight’’ is used to describe in-
fants 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 im-
provements in survival led to the need for further
classifications of LBW to better identify high risk in-
fants. 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 il-
lustrated 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 in-
fant 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
BIRTHWEIGHT  61FIGURE 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 restric-
tion are more readily understood than those of pre-
term 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 fac-
tors include multiple births (e.g., twins), infant gen-
der, and several factors relating to the mother,
including: birthweight, racial or ethnic origin, age,
height, infections, history of prior low birthweight de-
livery, work/physical activity, substance use/abuse,
cigarette smoking, alcohol consumption, and socio-
economic status. While prenatal care was once touted
as a highly effective means to prevent low birthweight,
more recent assessments have raised serious chal-
lenges 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 un-
derlying the risk of delivering a LBW infant. Socio-
economic status is linked to individual behaviors,
such as cigarette smoking and alcohol consumption,
and varies markedly by race and ethnicity. While so-
cioeconomic status and race/ethnicity cannot be
termed ‘‘causes’’ of low birthweight, they serve as in-
dicators of complex links between environmental,
psychological, and physiological factors that may re-
sult in higher risks of low birthweight.
The percentage of LBW infants in the United
States rose during the last two decades of the twenti-
eth 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 dis-
ease, 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, deaf-
ness, psychomotor problems, school failure, subnor-
mal growth, and health problems, which are
62 BIRTHWEIGHTcompounded by poverty and related adverse socio-
economic factors.
High Birthweight
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 exceed-
ing 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 diabe-
tes 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 usu-
ally 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 glu-
cose, a substance necessary for fetal growth, is elevat-
ed in both diabetic and obese women, these mothers
are more likely to have macrosomic births.
Risks for birth injuries rise rapidly for heavier ba-
bies, with vaginal deliveries being related to higher
morbidity and mortality for both the infant and the
mother. Lacerations of the birth canal and hemor-
rhaging 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.
See also: INFANT MORTALITY; PREMATURE INFANTS
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BIRTHWEIGHT  63World Health Organization. Manual of the International Statistical
Classification of Diseases, Injuries, and Causes of Death, Sixth Revi-
sion, Adopted 1948. Geneva: World Health Organization, 1948.
Martha Slay
Greg R. Alexander
Mary Ann Pas