Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005
Determinants of Low Birth Weight
Researchers have identified and analyzed many social, medical, and behavioral risk factors for low birth weight, some of which could contribute to racial disparities in low birth weight, and ultimately to school readiness. Many of these risk factors are intricately intertwined, and for the most part I will not attempt to establish or disentangle causal effects.
Women of low socioeconomic status are at increased risk for delivering low birth weight babies, whether socioeconomic status is defined by income, occupation, or education.55 Education may also have independent effects, above and beyond income, because more highly educated mothers may know more about family planning and healthy behaviors during pregnancy. In 1998, the rate of low birth weight among mothers with less than a high school education was 9 percent, as against 7.9 percent among high school graduates, and 6.5 percent among mothers with at least some college.56 In 2000, 78.6 percent of white women giving birth, and 74.5 percent of black women giving birth, had twelve or more years of education.57 Black Americans are much more likely than whites to come from a disadvantaged socioeconomic background, but that does not fully explain the racial disparity in low birth weight.58
Marital status is also a key correlate of birth weight. In 1992, the rate of low birth weight babies among unmarried mothers in the United States was 10.4 percent, as against 5.7 percent among married mothers.59 In 2000, 27.1 percent of low birth weight babies born to white mothers and 68.5 percent of low birth weight babies born to black mothers had unmarried parents.60 The marital status disparity may reflect either the greater likelihood of unmarried mothers to be poor or other characteristics that vary by marital status.61
In 2000, 19.7 percent of births to black women and 10.6 percent to white women in the United States were to teens. The rate of low birth weight babies among teen mothers was 35 percent higher than that among mothers aged twenty to twenty-nine (9.6 percent as against 7.1 percent). The rate among the youngest teens—those fifteen and younger— was 14.1 percent, higher than in any age group except forty-five to fifty-four.62 Teen mothers' birth weight disadvantage has several explanations. A pregnant teenager who is still growing may compete for nutrients with the fetus. Becoming pregnant within two years after menarche increases the risk for preterm delivery.63 Many teen pregnancies are unplanned, unwanted, or discovered late, and pregnant teens are more likely than older mothers to be poor, to be undereducated, or to lack access to resources or services—all, in themselves, risk factors for low birth weight.64
In 1992 Arline Geronimus found, surprisingly, that black teen mothers seem to have a paradoxical advantage in birth outcomes over older black mothers. She speculated that this finding may be due to “weathering” among black women—more rapid age-related deterioration in health than among white women because of greater cumulative exposure to harsh living conditions. Thus young maternal age may not be as much a risk factor among black mothers as it is among whites.65 Unadjusted national figures for black mothers from 2000 do not reflect this pattern; low birth weight rates among black mothers were lowest among mothers in their twenties.66 If the national sample were restricted to disadvantaged black mothers, however, the Geronimus weathering pattern might become apparent.
On the other end of the age spectrum, women who give birth in their late thirties or older are also at increased risk for having low birth weight babies. In 2000, 9.7 percent of births to black women and 13.9 percent of births to white women in the United States were to women aged thirty-five and over.67 For these women the risks are biological: older ova and a greater likelihood of medical risk factors such as hypertension.68 Older women also have more unintended pregnancies— itself a risk factor for low birth weight—than do women in their twenties and early thirties.69
One study found that women aged thirty and older are at greater risk for poor birth outcomes than teens of the same race, though offsetting factors such as higher socioecoomic status mask this risk.70 That same study, which controlled for such socioeconomic characteristics as whether the birth was covered by Medicaid, found evidence of the Geronimus weathering phenomenon. Black mothers aged fifteen to nineteen were at lower risk of delivering low birth weight babies than were black mothers in their twenties. Given the complicated relation between maternal age and low birth weight, it is difficult to assess the extent to which black mothers are at increased risk in this regard.
Among the medical risk factors for low birth weight and preterm birth are prior low birth weight or preterm delivery, cervical abnormalities, hypertension, anemia, and bacterial infections.71 Chronic physical or psychological stress also increases the risk.72 Among the risk factors for fetal growth retardation are previous low birth weight births, infections, sexually transmitted diseases, poor maternal hematological status, hypertension-related complications, renal disease, heart disease, third trimester bleeding, and sickle cell disease.73 Nutritional inadequacy can also impair fetal growth.74
Most, but not all, of these medical risk factors are more prevalent among blacks than whites. Most are rare. In 2000, for example, 3.8 percent of black mothers and 2.1 percent of white mothers were anemic during pregnancy; 1.4 percent of black mothers and 0.7 percent of white mothers had chronic hypertension. Black mothers had higher rates of acute or chronic lung disease, genital herpes, hydramnios or oligohydramnios (too little or too much amniotic fluid), hemoglobinopathy (a blood disorder), pregnancy-associated hypertension, eclampsia, incompetent cervix, and previous preterm babies or growthretarded infants. White mothers had higher rates of cardiac disease, renal disease, Rh sensitization, and uterine bleeding.75 Bacterial vaginosis, a mild bacterial infection more common among black women than white women, has been linked with preterm delivery of low birth weight infants.76
Prenatal Substance Use
Maternal cigarette smoking during pregnancy decreases fetal growth rates and substantially increases the risks of spontaneous abortion, preterm delivery, low birth weight, placental ruptures, placenta praevia, and perinatal death. Prenatal alcohol and drug use are also linked with poor birth outcomes, though the relationships are less clear-cut and not as dose-response specific as that of smoking.77 Substance abuse during pregnancy, particularly of alcohol and illicit drugs, is notoriously underreported. Based on reported rates of smoking, black mothers do not appear to be at increased risk for low birth weight. In 2000, 9.1 percent of black mothers and 13.2 percent of white mothers in the United States reported smoking cigarettes (at all) during pregnancy. The proportion of black and white mothers who reported consuming alcohol at all during pregnancy according to birth records in 2000 was virtually identical—about 1 percent of each group.78 However, these rates are nowhere near the proportion (16.3 percent) of pregnant women aged eighteen to forty-four who reported alcohol consumption in the past month in the 1995 Behavioral Risk Factor Surveillance System.79 For this reason, prenatal alcohol consumption has since been removed from the U.S Standard Certificate of Live Birth. In the 2001 National Household Survey on Drug Abuse, reported rates of current illicit drug use were similar among white (4.0 percent) and black (3.7 percent) pregnant women.80
Several studies have found strong associations between parents' (generally mothers') birth weight and the birth weight of their child.81 A recent study comparing maternal cousins (children whose mothers are sisters), and thus filtering out much of the confounding effect of socioeconomic status, found that maternal and paternal low birth weights together explain a much larger share of the racial disparity in low birth weight than do individual characteristics and socioeconomic variables combined.82 This finding suggests that there is a biological transmission of low birth weight across generations, which may contribute to racial differences in low birth weight. This is an important finding that can be used to target interventions, but given the strong association between birth weight and socioeconomic status, it should not be used to dismiss racial disparities as immutable.
Promising Directions for Future Research on Risk Factors
Other risk factors warrant further study and ultimately may offer strategies for reducing rates of low birth weight and narrowing racial disparities in low birth weight and school readiness. For the most part, research on these risks is in its infancy, and the associations being explored should not be interpreted as causal.
MATERNAL LIFESTYLE. Despite the beneficial effects of employment on income, mothers who work in strenuous occupations, including those that involve prolonged standing, are at heightened risk for both preterm delivery and having low birth weight babies.83 Occupational exposures to toxic substances and solvents have also been linked to preterm delivery.84 Given that a greater share of black women than white women (in 2002, 9 percent as against 5 percent) in the United States work as operators, fabricators, and laborers, black mothers may be more likely than white mothers to encounter strenuous working conditions and toxic exposures.85
NEIGHBORHOODS. Living in a poor neighborhood may pose health risks above and beyond those associated with individual poverty. Houses and other buildings in poor neighborhoods tend to be old and in poor condition; environmental toxins tend to be high; and access to medical care and other services tends to be limited.86
One study of Chicago in 1990 found that living in different neighborhoods accounted for as much as 30 percent of the difference in mean birth weight between non-Hispanic blacks and whites, though it is unclear whether these “neighborhood effects” reflect social, economic, or physical characteristics of neighborhoods or unobserved individual-level risk factors that vary by neighborhood.87 Neighborhood socioeconomic characteristics, such as census tract–level income, are important predictors of low birth weight, even after controlling for many individual-level characteristics.88 In Chicago, violent crime in neighborhoods has been found to have a negative association with birth weight, while a combined measure of social interaction and community involvement has a positive association.89 Many studies have linked low birth weight to residential environmental exposures, including air pollution, substances in drinking water, and industrial chemicals.90 Three-quarters of the residents of high-poverty neighborhoods in the United States are minorities, and the number of blacks living in poor areas increased from 2.4 million in 1970 to 4.2 million in 1990. Thus black women are at high risk for delivering low birth weight babies on the basis of the neighborhoods in which they live.91
PATERNAL FACTORS. Finally, a growing body of research suggests that paternal behaviors and occupational exposures before conception may affect infant health. Male reproductive toxicity can have three mechanisms— nongenetic (seminal fluid), genetic (gene mutations or chromosomal abnormalities), and epigenetic (effects on gene expression, genomic imprinting, or DNA methylation).92 One study linked paternal drinking and low birth weight, but its finding has not been replicated.93 Others have found associations between paternal smoking and low birth weight, although it is difficult to disentangle potential direct effects of paternal smoking from indirect effects through maternal exposure to secondhand smoke.94 Paternal occupational exposures are also a risk factor. Excess rates of preterm delivery, growth retardation, and low birth weight have been found in occupations that involve paternal exposure to pesticides, solvents, and lead.95 In 2002, 28 percent of employed black men, as against 16 percent of employed white men in the United States, worked as operators, fabricators, and laborers, perhaps making black fathers more likely than white fathers to be exposed to toxic substances at work.96