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OCR for page 4
4
addition to prolonged hospitalization at birth, almost 40 percent of very Tow
birthweight infants are rehospitalized more than once during the first year,
for an average of 16 days. This compares with 19 percent of all Tow birth-
weight infants for an average of 12.5 clays, ant! 8.7 percent of normal birth-
weight infants for an average of ~ clays.
The birth of a low birthweight infant and the problems that follow may
place substantial emotional and financial stress on a young family. The
effects of this stress on the well-being of the infant, on the parent-chiTc! bond,
and on siblings and the marital relationship are still being investigated.
Decreasing Mortality: The Effect on Morbidity
Greater success in saving the lives of low birthweight infants has not
increased our burclen of babies with handicaps, according to recent studies.
The proportion with severe congenital malformations or retarded develop-
ment remains the same, and there has been a decrease in the proportion of
those with less severe problems related to birth. Concern remains, however,
about the effects of increased survival of the very smallest infants, those less
than 1,000 grams. These survivors constitute a relatively new population that
will require long-term follow-up and evaluation.
CAUSES OF LOW BIRTHWEIGHT
An analysis of the causes of Tow birthweight must differentiate between
those responsible for premature birth and those leading to intrauterine
growth retardation (TUGR). Trying to separate the two conditions is compli-
cated, however, because {UGR and prematurity occur together in a sub-
stantial portion of low birthweight cases.
The physiological and biochemical events that initiate and maintain nor-
mal human labor are not well understoocI, although recent investigations
have producecl important new clues about hormonal factors in the process.
Theoretical models based on these clues have allowed researchers to begin
studying variations from the normal pattern that might lead to premature
labor. Certain clinical conditions, discussed in the following section on risk
factors, appear to cause changes in the hormonal environment and metabolic
state of the uterus and cervix. These changes probably result from complex
interactions involving progesterone, estrogen, oxytocin, and other hor-
mones; prostaglandins; calcium ions; adrenergic agents and receptors;
catecholamines; and uteroplacental blood flow.
lUGR is associated with conditions that interfere with the blood circulation
to and efficiency of the placenta, with the development of the fetus, or with
the general health and nutrition of the pregnant woman. In many cases,
however, no relevant pathogenic factors can be found. Maternal vascular
diseases, such as chronic hypertension, chronic renal disease, or sickle-cell
disease, may hamper delivery of nutrients or oxygen to the fetus. Multiple
pregnancies may result in fUGR because the placenta cannot supply suffi-
cient nutrients to more than one fetus. Fetal factors associated with JUGR
include chromosomal disorders, chronic fetal infections such as congenital
rubella and syphilis, and radiation injury. All of these associations suggest
OCR for page 5
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possible pathogenic mechanisms, but the underlying physiological pro-
cesses have not been identified.
Risk Factors
In the absence of adequate information about the basic causes of low
birthweight, a large body of information has cleveloped about "risk factors,"
or factors whose presence in an individual woman indicates an increased
chance of bearing a low birthweight infant. These factors, which will be
outlined further in the following pages, are listed in Table I. They include
demographic characteristics, medical risks that can be iclentifiecT before preg-
nancy and those that can only be identified during pregnancy, behavioral
and environmental factors, risks associated with health care (such as inade-
quate prenatal care), and a separate group of factors whose relationship to
low birthweight is more tenuous, such as stress, uterine irritability, and
inadequate plasma volume expansion.
Grouping the risk factors as noted on the table helped the committee
identify those that can be detected before pregnancy and reinforced the
concept that interventions can begin before the prenatal period. Smoking is
perhaps the best example of this perspective. The grouping also emphasizes
the importance of behavioral and environmental risks and the need for
interventions that go beyond medical care. The demographic measures can
help to define target populations. The cluster of health care factors highlights
the fact that not all risks for low birthweight derive from characteristics of
OCR for page 6
6
women themselves. And finally, the category of evolving concepts of risk
suggests some important research areas. These themes appear throughout
the main report.
The committee concluded that a variety of factors are clearly and con-
sistently linked to low birthweight. These factors should be used to help
define high-risk groups and to develop and target interventions. It is appar-
ent, however, that the magnitude of risk posed by each factor for an individ-
ual or for a group cannot always be calculated easily, that the risks for Tow
birthweight are widely distributed throughout the population, and that a
substantial number of low birthweight cleliveries will continue to occur
outside of groups currently clefined as high risk. These circumstances high-
light the need for greater understanding of risk and causation, but should not
be used to minimize the value of using existing risk information for targeting
interventions.
Demographic Risks
Because the major demographic risk factors are often interrelated, it has
been difficult to determine the precise association between any single factor
and low birthweight. Nonetheless, careful research is graclually defining the
independent effects of many factors.
Race Black newborns are more than twice as likely to weigh less than
2,500 grams as white infants. The race-specific low birthweight rates among
live births in the United States in 1981 were 12.5 percent for blacks and 5.7
percent for whites. The reasons for the higher risk among blacks are not clear.
It has been speculatecl that maternal age may account for part of the
difference twice as many black births are to teenagers but when black and
white mothers of the same age are compared, blacks are at higher risk of Tow
birthweight in every age group. Similarly, black mothers as a group have less
education that white mothers, but when blacks and whites are matched by
level of education, blacks still have a higher risk of low birthweight. Other
factors that have been stuclied but fad! to account for the white-black differen-
tial inclucle delay in initiating prenatal care, smoking status, height and
weight distributions of the mother, and obstetric history.
The committee's analysis of national and statewide trends in the white-
black differential in low birthweight indicates that the gap is not closing. For
the United States as a whole, the relative decline in white Tow birthweight
rates between 1971 and 1981 exceeded the corresponding relative decline in
black low birthweight rates. White Tow birthweight rates cleclined by 14
percent between 1971 and 1981, while black rates declined by only 6 percent.
The absolute declines among whites and blacks, however, were more com-
parable. It appears, therefore, that characterizing trends in the difference
between black and white rates of Tow birthweight depends to some extent on
the measures used.
The issue of race and Tow birthweight is further complicated by the differ-
ent birthweight-specific neonatal mortality rates of white and black infants.
Black infants weighing less than 2,500 grams have Tong been recognized to
have better rates of survival in the neonatal period than white infants of
OCR for page 7
7
TABLE 1 Principal Risk Factors for Low Birthweight
I. DEMOGRAPHIC RISKS
A. Age (less than 17; over 34)
B. Race (black)
C. Low socioeconomic status
D. Unmarried
E. Low level of education
II. MEDICAL RISKS PREDATING
PREGNANCY
A. Parity (0 or more than 4)
B. Low weight for height
C. Genitourinary anomalies/surgery
D. Selected diseases such as diabetes,
chronic hypertension
E. Nonimmune status for selected
infections such as rubella
F. Poor obstetric history including
previous low birthweight infant,
multiple spontaneous abortions
G. Maternal genetic factors (such as
low maternal weight at own birth)
III. MEDICAL RISKS IN CURRENT
PREGNANCY
A. Multiple pregnancy
B. Poor weight gain
C. Short interpregnancy interval
D. Hypotension
E. Hypertension/preeclampsia/toxemia
F. Selected infections such as
symptomatic bacteriuria, rubella,
and cytomegalovirus
G. 1st or 2nd trimester bleeding
H. Placental problems such as placenta
previa, abruptio placentae
I. Hyperemesis
I. Oligohydramnios/polyhydramnios
K. Anemia/abnormal hemoglobin
L. Isoimunization
M. Fetal anomalies
N. Incompetent cervix
O. Spontaneous premature rupture of
membranes
IV. BEHAVIORAL AND
ENVIRONMENTAL RISKS
A. Smoking
B. Poor nutritional status
C. Alcohol and other substance abuse
D. DES exposure and other toxic
exposures, including occupational
hazards
E. High altitude
V. HEALTH CARE RISKS
A. Absent or inadequate prenatal care
B. Iatrogenic prematurity
VI. EVOLVING CONCEPTS OF RISK
A. Stress, physical and psychosocial
B. Uterine irritability
C. Events triggering uterine
contractions
D. Cervical changes detected before
onset of labor
E. Selected infections such as
mycoplasma and Chlamydia
trachomatis
F. Inadequate plasma volume
expansion
G. Progesterone deficiency
similar birthweight. Based on this observation, some researchers have sug-
gested that the 2,500-gram marker of low birthweight may not have the same
implications for nonwhite infants as for whites; however, this line of reason-
ing is overshadowed by the more imposing fact that nonwhite infants are
twice as likely to be born at low birthweight and twice as likely to die in the
neonatal period as white infants.
The conclusion to be drawn from the complicated ciata on race, low
birthweight, and race-specific birthweight mortality rates is that the reasons
for the risk differentials between white and black newborns are not well
understood. The cumulative effects over time of black poverty and lower
social status, and the interaction of such factors with biological processes,
undoubtedly have played a role in these racial differences; other factors
remain to be defined. Research should be pursuer! to improve our under-
standing of these important issues.
OCR for page 8
8
Age Teenage mothers and those age 35 years or older have higher rates of
low birthweight than mothers in their twenties and early thirties. The risk is
highest for young mothers, especially among whites; however, childbearing
in the teen years is more prevalent among blacks. In 1980, 211 percent of all
black births were to teenagers, compared with 12.! percent of white births
and 15.3 percent of Hispanic births.
Studies indicate that young age is not an independent risk factor for Tow
birthweight. Teenage mothers have many other characteristics that increase
the likelihood of a low-weight birth. They are more likely to be black, of low
socioeconomic status, and unmarried than older mothers. Also, they are
shorter and lighter, less educated, and more likely to report late for prenatal
care than their older counterparts.
Socioeconomic Status Tow socioeconomic status (SES), measured in terms
of social class, income, education, or census tract, is clearly associated with
an increased risk of preterm delivery and TUGR. The literature suggests that
at least some of the excess risk arises from separate factors linked both to low
social class and low birthweight. These include smoking, low maternal
weight gain and short stature, obstetric complications such as hypertension
and preecIampsia (a toxic condition of late pregnancy), some types of
genitourinary tract infections, and limited access to high-quality prenatal
care. The effect of socioeconomic status probably represents the sum of many
factors, each of which may increase the risk of poor pregnancy outcomes.
Education The risk of low birthweight declines sharply among mothers
with at least 12 years of education. The relationship between education and
low birthweight is independent of maternal age and race. The committee's
analysis of national data indicates that the gap in Tow birthweight rates
among mothers with different levels of education is not closing and may be
widening. This finding is especially important given that the educational
attainment of mothers has increased significantly during the past 10 to 15
years. The widening gap suggests that the poorly educated may constitute
an increasingly high-risk group.
Marital Status Unmarried mothers have a consistently higher risk of
bearing a low birthweight infant than those who are married. This risk is not
attributable to differences in age or race. In 1980, the Tow birthweight rate for
infants born to unmarried mothers was Il.6 percent, compared with 5.8
percent for babies whose mothers were married. The significance of marital
status as a risk factor is underscored by the increase in childbearing among
unmarried women. Between 1976 and 1981, the proportion of white mothers
reported to be unmarried increased from 7 percent to 12 percent; for blacks,
the proportion increased from 51 percent to 56 percent.
Medical and Obstetric Risks
Medical and obstetric risks for low birthweight can be divided between
those detectable before pregnancy, such as chronic illness in the mother or a
history of poor pregnancy outcome, and those that can be noted only during
pregnancy, such as placenta previa. The committee focused on a subset of
OCR for page 9
9
these problems: hypertension/preecIampsia, diabetes, obstetric history (in-
clud~ng previous Induced abortion), multiple pregnancy, and infection.
Hypertens ion/PreecZa mps ia
Hypertension is the disease most often associ-
ated with fetal growth retardation; it also can be associated with preterm
delivery. In one study population in the 1970s, researchers found that 27
percent of lUGR with an identifiable cause could be attributed to severe
preecIampsia, chronic hypertensive vascular disease, or chronic renal dis-
ease.2 Infants with TUGR were born to 30 percent of patients with a diagnosis
of chronic hypertension and to 46 percent of patients with severe preecIamp-
sia. Elevated maternal blood pressure also may cause preterm labor (often
precipitated by premature detachment of the placenta), or necessitate medi-
cal intervention to deliver the baby and thereby avoid more serious prob-
lems.
Diabetes Maternal diabetes mellitus is frequently associated with babies
born large for gestational age, but the disease also may lead to JUGR or
preterm delivery. In the past, diabetes-related premature births often re-
sulted from physician interventions to avoid unexpected intrauterine death.
These early deliveries were appropriate in some cases but not in others.
Improved management of diabetes and new techniques to assess fetal well-
being, gestational age, and lung maturity have reduced the number of
unnecessary early deliveries associated with maternal diabetes. Premature
delivery may still be necessary in some cases, however, for pregnant women
with insulin-dependent diabetes complicated by diabetic vasculopathy. In
such women, preterm delivery may be required because of worsening mater-
nal retinopathy, nephropathy, or Hypertension.
Increasing evidence indicates that poor control of maternal diabetes dur-
ing the early weeks of embryonic development may contribute both to poor
fetal growth and congenital defects. Researchers have shown that excellent
control of diabetes before conception and during the early weeks of pregnan-
cy can decrease the risk of fetal malformation,3 but the effects of such control
on fetal growth have not been explored.
Obstetric History The history of a woman previous pregnancies is of
prime importance in the prediction of a subsequent Tow birthweight infant. A
detailed study of the weights and gestational ages of all births in Norway
from 1967 through 1973 showed that a premature first birth is the best
predictor of a premature second birth and that growth r~t~r~l~tinn in ~ [irct
. .
_ _ O ~ % ~ . . ~ ~ ~ ~ ~7 .
pregnancy IS the most powerful predictor of growth retardation in a second
pregnancy.4 Previous fetal and neonatal deaths also are strongly associated
with preterm low birthweight, and the risk increases as the number of poor
fetal outcomes increases.
The effect of the interaction between maternal age and birth order on Tow
birthweight has been well documented. The incidence of low birthwei~ht is
1~ ~1_ r~ 1 . ~ _ ~ ~ ~ ~
Sign for women Between ~ ~ and ~v bearing their second or later child, Tow for
women age 25 to 34 bearing their third or later child, and increases sharply
among women having their first child after age 29.
Special tabulations on 1981 live births performed for the committee by the
National Center for Health Statistics show that interval between pregnancies
OCR for page 10
10
also affects low birthweight. The risk is sharply elevated for an interval of less
than 6 months, decreases moderately from 6 months to between 24 months
and 36 months, and then rises gradually. However, short interpregnancy
interval is not associated with an increased risk of Tow birthweight if the
previous pregnancy ended in a fetal death.
Previous Induced Abortion Because about 1,500,000 legal induced abor-
tions occur annually in the United States, the committee believed it was
important to assess the impact of such procedures on the incidence of {UGR
and preterm delivery in subsequent pregnancies. A review of the literature
showed that:
· the risk of Tow birthweight or preterm delivery in a pregnancy following
one that is terminated by vacuum aspiration abortion in the first trimester
(the most common abortion procedure in the United States) is no greater
than the risk of adverse outcome expected for a first pregnancy; and
· the effect of multiple abortions on subsequent pregnancies is unclear;
some studies have shown an increased risk of Tow birthweight and others
have not the outcome may depend on the type of abortion procedure
performed. There is some concern that abortion techniques requiring cervi-
cal dilatation of more than 12 millimeters may lead to problems of cervical
incompetence, and therefore an increased risk of prematurity in subsequent
pregnancies.
The committee concluded that research is needed to investigate further the
relationship of induced abortion to the outcome of future pregnancies.
MuZtipZe Pregnancy Pregnancies with twins, triplets, or more carry an
increased risk both of preterm delivery and low birthweight. Even at full
term, infants in plural deliveries are T! times more likely to be of Tow
birthweight than singleton deliveries. Neonatal mortality also is greatly
increased in multiple pregnancies, and morbidity is high among survivors.
Infections A variety of infections have been linked with both preterm
delivery and intrauterine growth retardation. For some infectious agents, a
causal role in Tow birthweight has been established; for others, the associa-
tion is less clear. Many of these infections can be prevented or treated,
reducing the risk of an adverse pregnancy outcome.
Congenital rubella syndrome and cytomegalovirus infection, both
commonly associated with congenital defects, also can cause intra-uterine
growth retardation. The incidence of rubella virus infection in the United
States and the number of congenitally infected newborns have clecreased
significantly since the introduction of the rubella vaccine.
Untreated or inadequately treated symptomatic urinary tract infections are
known to cause a variety of problems for both mother and fetus, including
Tow birthweight. Pregnant women with no symptoms of such infections
(asymptomatic) should be screened routinely for bacteriuria (the presence of
bacteria in the urine), because symptomatic infections can be prevented in
most patients by treatment of asymptomatic infections. Culture techniques
are inexpensive anct easy to use; patients with known infections can be
taught to culture their urine at home.
Another organism that may leac! to Tow birthweight as a result of maternal
OCR for page 11
11
genitourinary infection is mycoplasma. In one recent study, mycoplasma-
infected women treated with a 6-week course of erythromycin showed a
markedly recluced incidence of low birthweight.5 More research is needed to
confirm this effect and to explore its significance.
Some researchers believe that certain genital pathogens may trigger pre-
term labor, as well as affect intrnl]tf~rino Growth To ~nlr~r~o this role_
I- _ _ _1 · ~.1 ~
en ~ ^ ~ ~^ ~ ~ ~_~ ~ ~ ~ ~ ~ ~ V ~ ~1 L. 1 V ~A~lW1 ~ L1 [~= 1 Elm
t~onsn~ps further, future studies must examine comprehensively the flora of
the cervix and vagina, identify local and systemic immune responses, and
assess their combined influence on pregnancy outcome.
Nutrition
Four types of research have been used to examine the effect of nutrition
during pregnancy on birth outcomes: animal stuclies, human war famine
studies, nutritional intake/fetal outcome correlational studies, and ex-
perimental nutrition intervention studies. They all point to the common
conclusion that good nutrition has a positive influence on birthweight, but
the extent of the effect is unclear. The magnitude of nutritional effects on low
birthweight is not easily assessed because nutritional status is difficult to
isolate from other socioeconomic characteristics and because of the compli-
cated relationship between prepregnant weight and weight gain during
pregnancy. While researchers have found positive relationships between
birthweight and nutritional status, there is wide variability in the degree of
these associations. A reasonable conclusion is that poor nutritional status
before pregnancy and inadequate nutrition during pregnancy have a nega-
tive impact on fetal weight gain, thereby increasing the risk of JUGR.
One recent study explored the relationship between a mother's weight
gain during pregnancy and the occurrence of low birthweight by analyzing
data from the 1980 National Natality and Fetal Mortality Surveys. The inves-
tigators found that many groups of women known to have an increased risk
of delivering a low birthweight infant also were more likely to have inade-
quate weight gains. For example, they found that black mothers were twice
as likely as white mothers to gain less than 16 pounds during pregnancy. In
addition, mothers 35 years of age or older and teenage girls were less likely to
gain at least 16 pounds, as were unmarried women, poorly educated
women, anc! women of lower socioeconomic status. A further analysis of
numerous risk factors among white mothers only indicated that, except for
period of gestation, weight gain has the strongest impact on birthweight.6
Behavioral and Environmental Risks
Smoking Smoking is one of the most important and preventable cletermi-
nants of low birthweight in the United States. A recent survey of the litera-
ture on smoking and birthweight indicates that smoking during pregnancy is
associated with a reduction in birthweight ranging from 150 to 250 grams.
This relationship has persisted for at least 20 years, despite reported reduc-
tions in the average tar and nicotine yields of cigarettes on the market.7 The
reasons for the detrimental effects of cigarette smoking are not fully under-
stood, but the fact that an estimated 20 to 30 percent of pregnant women in
the United States smoke underscores the importance of this risk factor.
OCR for page 12
12
Alcohol Use The data on maternal alcohol consumption and its association
with low birthweight are not as uniform as for smoking. it is reasonably
certain that pregnant women who drink heavily are at risk of delivering a
baby with fetal alcohol syndrome characterized by lUGR and a variety of
other problems. The impact of moderate alcohol use is less clear. A 1983
review of the literature suggested that regular drinking of fewer than two
drinks per day probably is not an important determinant of JUGR,7 but two
more recent studies contradict this conclusion. In one, a prospective study of
30,000 pregnancies, women who drank one or two drinks per day proved to
have an increased risk of {UGR even after the figures were adjusted for
maternal age, race, education, marital status, and a variety of other risk
factors.8
Uncertainty over the effects of alcohol on fetal development warrants
caution: the Surgeon General of the United States has advised pregnant
women not to drink alcoholic beverages.
latrogenic Risks
{atrogenic prematurity refers to the birth of a physiologically immature
and/or low-weight infant who is delivered prematurely as a result of medical
intervention. Some cases are justifiable-the decision to end a pregnancy
early may be made to avert more serious consequences for the mother or
infant but others are practitioners' mistakes. Studies conducted in the early
and micI-1970s found that from 4 to ~ percent of infants admittect to neonatal
intensive care units in three cities had been born prematurely as a result of
labor inductions and electively timed cesarean sections.9~~
Accurate prenatal assessment of gestational age, combined with selected
use of ultrasound examination anc! new techniques to test fetal lung matur-
ity, could reduce the number of cases of accidental iatrogenic prematurity.
{atrogenic prematurity also could be reduced by decreasing the number of
primary and repeat cesarean sections.
Evolving Concepts of Risk
A desire to improve the health care professional's ability to identify preg-
nant women at risk of a low-weight birth has lect researchers to study a
variety of other possible risk factors. Those described in the full report
include stress, uterine irritability anct the notion of "triggering factors,"
certain cervical changes cletected before labor begins, inadequate plasma
volume expansion, and progesterone deficiency. The first three factors only
are outlined below.
Stress The relationship between socioeconomic status and Tow birth-
weight suggests that a woman's response to her environment may have an
impact on pregnancy outcome; it may be, for example, that poverty is a risk
factor for low birthweight because of the high levels of stress associated with
being poor. Two types of stress have been examined in numerous studies:
physical stress and fatigue, particularly as related to employment during
pregnancy, and psychological distress resulting from maternal attitudes
toward the pregnancy or from external stressors in the environment.
Major studies of physical stress (usually in a work-related setting) indicate
OCR for page 13
13
that there may be some association
between low birthweight (both {UGR
anct prematurity) and activities that
require long periods of standing or
other significant amounts of physical
stress. There is no conclusive evi- o
dence, however, that maternal em- ~
ployment per se increases the risk of °
Tow birthweight. ,:
Numerous reports suggest a link
between stress and conditions that ~-
increase the risk of JUGR and pre- x
term labor, such as preecIampsia, but ~
relatively little ciata exist to support a O
strong, direct relationship between
maternal psychological stress and
low birthweight. Some data link psy-
chological stress to a number of other
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poor pregnancy outcomes such as fetal distress, neonatal motor immaturity,
perinatal deaths, and congenital anomalies.
Research in this area is plagued by a variety of problems, including the
absence of a clear measure, or marker, of stress. Many studies suffer from a
major methodological flaw-the stress is not assessed until after the event.
This introduces the potential for recall bias, which could increase the report-
ing of stress among mothers with poor pregnancy outcomes. Also, concepts
and definitions of stress vary substantially among studies, and sample sizes
are often too small to isolate specific outcomes such as low birthweight.
Finally, most current projects evaluating the effects of stress fail to control for
smoking, a major correlate of low birthweight.
Uterine Irritability The concept of uterine irritability and the possibility
that certain external factors can stimulate or "trigger" uterine contractions
are just beginning to be explored. Some studies suggest that excessive
uterine activity may be a component of the events leading to preterm labor.
For example, in a recent study comparing the rhythm of uterine contractions
in the latter part of pregnancy over a 24-hour period in normal pregnancies
and in pregnant women at high risk of preterm labor, the women with
normal pregnancies were found to have long periods during which the
uterus was quiet each night. In contrast, the high-risk women had uterine
activity throughout the 24-hour cycle. Assessing the extent of uterine
activity could be part of the surveillance of pregnant women at risk of
preterm labor. Similarly, it may prove prudent to advise high-risk women to
avoid those activities (which vary among individual women) that stimulate
uterine contractions.
Cervical Changes Recently, interest has grown in increasing the number of
cervical assessments made in later pregnancy in order to identify changes
that occur several days and sometimes even weeks before the onset of
preterm labor. The presence of a short cervix or dilatation may lead the
clinician to initiate some form of intervention to forestall preterm labor. Some
prenatal care specialists advocate regular pelvic exams in later pregnancy to
OCR for page 14
14
detect these changes, but others have expressed concern that these exams
might lead to premature rupture of the membranes or other problems. No
conclusive data exist on the risks of frequent cervical exams prior to term.
Risk Assessment
In an effort to use risk factor data to help structure prenatal care for
pregnant women, researchers have developed a variety of techniques to
measure risk status, including scoring systems (risk assessment instru-
ments) to distinguish women at high risk of preterm labor and/or TUGR from
women at Tow risk. The committee examined the predictive capabilities of 13
risk classification systems; a complete description of the results appears in
the main report. An important finding was that the majority of these systems
correctly identify as high risk about 65 percent or more of those pregnancies
with eventual adverse outcomes.
In addition to distinguishing high-risk from Tow-risk women, well-
constructect risk assessment systems have the potential to reduce the misdi-
agnosis both of {UGR anc! preterm labor and are helpful additions to clinical
judgement in evaluating the risk of low birthweight. They also offer the
possibility of grouping risk factors by their preventability or modifiability,
thereby suggesting possible interventions. And risk asessment systems can
encourage more appropriate referrals for care and more reasonable resource
allocations for the management of preterm birth.
The limitations of these instruments also must be recognized. First, be-
cause the performance of a risk assessment instrument is to some extent
dependent on the prevalence in a population of the adverse outcome being
assessed, it is unlikely to produce the same results in every setting. Second,
the instrument is a statement of probability only and cannot be viewed as a
definitive predictor for a specific woman. Third, the instrument cannot be
used in a rote manner to substitute for high-quality professional care. The not
infrequent occurrence of Tow birthweight deliveries in Tow-risk women sug-
gests that additional research is needed to improve the predictive capability
of these systems. It also indicates that clinicians must be alert to the possibil-
ity of Tow birthweight even in pregnant women judged to be at low risk of
such an outcome.
Research
A major theme that emerges from the voluminous information on the
causes and risks of low birthweight is the critical need for additional research
in many areas. In particular, our understanding of the physiological pro-
cesses involved in premature labor and JUGR is seriously inadequate. Efforts
to prevent Tow birthweight will remain limited until more is known about
basic causal mechanisms.
In addition, more research is needed on specific risk factors, not only those
somewhat speculative in nature, but also those clearly linkecl to low birth-
weight. For example, little is known about the ways in which race exerts an
effect on birthweight, and prevention strategies aimed at certain other risk
factors, such as alcohol abuse, could be improved if there were a better
definition of the magnitude of risk at various levels of consumption. Studies
Representative terms from entire chapter:
tow birthweight
