Bibliography
Cunningham, F. Gary, Paul C. MacDonald, Norman F. Gant, Ken-
neth J. Leveno, Larry C. Gilstrap, Gary D. Hankins, and Ste-
ven L. Clark, eds. Williams Obstetrics. Stamford, CT: Simon
and Schuster, 1997.
Doshi, M. L. ‘‘Accuracy of Consumer Performed In-Home Tests for
Early Pregnancy Detection.’’ American Journal of Public Health
76 (1986):512–514.
Gabbe, Steven G., Jennifer R. Niebyl, and Joseph L. Simpson, eds.
Obstetrics: Normal and Problem Pregnancies. New York: Chur-
chill and Livingstone, 1997.
Jeng, L. L., R. M. Moore, R. G. Kaczmarek, P. J. Placek, and R. A.
Bright. ‘‘How Frequently Are Home Pregnancy Tests Used?
Results from the 1988 National Maternal and Infant Health
Survey.’’ Birth 18 (1991):11–13.
National Research Council. Recommended Dietary Allowances, 10th
edition. Washington, DC: National Academy Press, 1989.
Valanis, B. G., and C. S. Perlman. ‘‘Home Pregnancy Testing Kits:
Prevalence of Use, False-Negative Rates, and Compliance
with Instructions.’’ American Journal of Public Health 72
(1982):1034–1036.
Garrett Lam
PREMATURE INFANTS
Premature infants are those who are born too soon,
that is, born before the normal length of time in preg-
nancy that is typically needed for a fetus to develop,
mature, and thrive postnatally. The average length of
pregnancy is thirty-nine to forty weeks, which is the
approximate duration of pregnancy needed for the
fetus to reach full development and maturity. Infants
delivered between the thirty-seventh and forty-first
week of pregnancy are typically referred to as ‘‘term’’
or ‘‘mature’’ births. ‘‘Preterm’’ or ‘‘premature’’ birth
is defined as delivery before the thirty-seventh week
of pregnancy and ‘‘postterm’’ or ‘‘postmature’’ births
are those occurring at forty-two weeks and beyond.
Preterm deliveries are further delineated as either
‘‘very preterm’’ (before the thirty-third week) or
‘‘moderately preterm’’ (between the thirty-third and
thirty-sixth weeks).
The use of the expression ‘‘prematurity’’ has
changed over time. It once was used to refer to any
early or small birth, thereby encompassing births that
occurred before term and births of infants of low
birthweight (less than 2,500 grams [5 pounds, 8
ounces]). It became apparent, however, that not all
low birthweight infants were born preterm and that
not all preterm births were low birthweight. Many low
birthweight infants are term births that are small in
size due to growth-related complications. These two
different types of low birthweight infants—those pre-
term and those small for their duration of gestation—
were recognized to reflect distinct medical problems,
and as a result, more specific labels were needed to
refer to each type of birth according to its birth-
weight, duration of gestation, and birthweight for
gestation. It is now the convention to clearly distin-
guish between births that are preterm, low birth-
weight, and/or ‘‘small for gestational age.’’ For single
live births born to U.S. resident mothers between
1995 and 1997, 9.6 percent were preterm and 6.1
percent were low birthweight. Of these low birth-
weight deliveries, 61.9 percent were preterm. Simul-
taneously, only 39 percent of preterm births were low
birthweight.
The duration of the pregnancy prior to delivery
in completed weeks is referred to as the gestational
age of the newborn and thereby establishes if the de-
livery is preterm. The duration of pregnancy is tradi-
tionally measured as the interval from the date of the
mother’s last menstrual period to the date of birth.
This approach for defining gestational age at delivery
is derived from obstetrical practice and overestimates
by approximately two weeks the interval from concep-
tion to birth. Alternate approaches to estimating the
gestational age have been developed and include
physical and neurological assessments of the newborn
and prenatal ultrasound measures of fetal size (body
length, femur length, and skull diameter and circum-
ference).
Knowledge of gestational age is essential for the
appropriate medical management of both the preg-
nancy and the newborn infant as gestational age
serves as a proxy measure for the extent of fetal devel-
opment and the fetus’s readiness for birth. As an indi-
cator of newborn maturity, gestational age is closely
associated with the newborn’s chances for survival
during the first year and the likelihood of developing
neonatal complications. Moreover, knowledge of a
preterm infant’s gestational age is necessary for as-
sessing developmental progress in infancy.
Gestational age is also used by public health pro-
fessionals to calculate a variety of statistical indicators
that are useful for monitoring the health status of
populations and assessing the need for and impact of
targeted public health interventions. Preterm and
very preterm percentages in populations may reflect
the prevalence of a variety of health-related concerns,
including infections, psychosocial and physical stress-
es, poor nutrition, and substance abuse.
Between 1981 and 1996 in the United States, an-
nual preterm rates rose from 9.4 percent to 11 per-
cent of live births, a 17 percent increase (see Figure
1). Although infant mortality rates declined during
the same period, the ongoing increase in preterm
rates is a matter of considerable concern. Indeed, in-
creasing rates of preterm birth are recognized as a
pregnancy related crisis in the United States. Approx-
imately three-quarters of neonatal deaths and nearly
one-half of the long-term neurological damage seen322 PREMATURE INFANTS