BIRTHWEIGHT
Birthweight is an important indicator of the approxi-
mate 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%) con-
centrated 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 mor-
tality soars (see Figure 1), although in recent decades,
heavier infant births have become less associated with
high mortality risks, probably because of medical in-
tervention. 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 in-
fant 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 tem-
poral, geographic, socioeconomic, hospital, or other
contrasts, infant mortality differences are typically ex-
amined 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 distribu-
tion of birthweights within categories (e.g., fewer
births at extreme birthweights), as opposed to reduc-
tions in birthweight-specific mortality rates (e.g., in-
fants 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 twenti-
eth century was due to improvements in survival rath-
er 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 medi-
cal technology to reduce infant death rates. Accord-
ingly, research attention has been directed at finding
the determinants of low birthweight in order to devel-
op more cost-effective, population-wide programs to
further diminish infant mortality.
Variations of average birthweight have been asso-
ciated with infant gender, multiple birth factors, and
maternal factors, such as race and ethnicity, size, nu-
trition, and current and previous pregnancy medical
risk characteristics. One of the unresolved questions
among researchers is whether there is a single com-
mon average human birthweight or whether there are
normal variations in average birthweight among pop-
ulation subgroups. This question entails important
medical care, public health policy, and political as-
pects 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.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 extensiveBIRTHWEIGHT 61