The Future of Children, Princeton - Brookings: Providing research and analysis to promote effective policies and programs for children.

The Causes of Low Birth Weight and Preterm Birth

To prevent low birth weight and preterm birth, it is necessary to know what causes them and to determine the modifiable factors that are highly related to these causes. Alexander and Korenbrot emphasize that very little is known about the causes of low birth weight and preterm birth despite an extensive amount of research. Low birth weight that results from suboptimal intrauterine growth is associated with three major risk factors: cigarette smoking during pregnancy, low maternal weight gain, and low prepregnancy weight. These three risk factors account for nearly two-thirds of all growth-retarded infants.⁵ Other risk factors for low birth weight include black race, first births, female sex, short maternal stature, maternal low birth weight, prior low birth weight birth, maternal illnesses, fetal infections, and a variety of metabolic and genetic disorders. While these risk factors may provide important clues about the causes of low birth weight, many of them are only weakly related to low birth weight and are generally not modifiable by intervention programs or changes in public policy. Even less is known about the risk factors for early delivery. While there are several known risk factors (cigarette smoking during pregnancy, prior preterm birth, low prepregnancy weight, maternal exposure to diethylstilbestrol [DES]), they account for fewer than one-quarter of preterm births.⁵

Translating the current epidemiologic knowledge about the causes of low birth weight and preterm birth into practical preventive care has been difficult and slow. Despite clear and convincing evidence that smoking cessation during pregnancy can prevent growth retardation and low birth weight, intensive smoking cessation interventions are not a routine part of most prenatal care programs. Nutritional interventions do not have as strong or clear an impact on low birth weight as smoking cessation. Even if effective ways could be found to prevent low prepregnancy weight and to promote adequate weight gain, these interventions would help only the small subset of women who are severely undernourished.

One of the more important clues about the causes of low birth weight and preterm birth comes from the observation that there are large ethnic group differences in these outcomes. African-American infants are two times more likely to be born low birth weight or preterm than are white or Asian-American infants. Epidemiologic research has shown that these ethnic group differences are not wholly explained by ethnic differences in the occurrence of various medical conditions, in smoking or use of other licit or illicit drugs, or in use of prenatal care, or by demographic characteristics and other lifestyle differences. Research to discover the reasons for these curious ethnic group differences may shed some light on the causes of low birth weight and preterm birth as well.

Medical care has been very successful in rescuing the low birth weight infant. However, much less progress has been made in finding solutions to prevent preterm labor or low birth weight. It is logical to think that monitoring the course of pregnancy by providing prenatal medical care might be one means of prevention.

Prenatal Care

Several national commissions have convened experts from many disciplines to determine how to prevent low birth weight and infant mortality.^6-8 These commissions have focused on one promising area—prenatal care. Prenatal care varies tremendously but usually includes a package of medical care services in a defined schedule of visits. In addition to medical care, prenatal care programs often include comprehensive educational, social, and nutritional services. While the structure and content of prenatal care was not developed primarily to prevent low birth weight, some data indicate that women who receive prenatal care have fewer low birth weight babies and lower infant mortality. The effectiveness of prenatal care and its components has not been adequately evaluated, and the benefits of this form of care have not been fully measured.⁹ Experimental studies have not been done primarily because it is not ethical to make random assignments of pregnant women to a control group that does not receive any prenatal care.

Recent analyses by Alexander and Korenbrot in this issue cast doubt on the degree to which prenatal care is truly effective in preventing low birth weight; self-selection may have produced the apparent advantage. Women who receive prenatal care are a heterogeneous group, but are generally healthier, more educated, and more advantaged than women who do not receive prenatal care. It is unclear whether or not the observed decreased rates of low birth weight among women who receive prenatal care are due to the effectiveness of prenatal care in preventing low birth weight or are due to other differences between women who receive prenatal care and those who do not. Rather than preventing low birth weight, prenatal care may be another indication of the many health-enhancing behaviors that characterize healthy, insured women who have healthy children. Although prenatal care provides a variety of valuable medical services to pregnant women and fetuses (for example, management of normal birthing, maternal hypertension, and diabetes in the mother and/or detection of congenital malformations and genetic diseases in the fetus) evidence suggests that prenatal care may not provide significant benefits with respect to low birth weight and preterm birth.

Nationally, there has been an increase in the receipt of early prenatal care in the past 20 years. In 1975, 72% of women started prenatal care in the first trimester of pregnancy.⁷ This number rose to 76% in 1980, remained stable throughout the 1980s, and increased to 78% in 1992.^7,10 Even more notable is that over 98% of all pregnant women currently receive some form of prenatal care.¹⁰ If use of prenatal care actually did prevent low birth weight, then one would expect a decrease in low birth weight with increasing use of prenatal care over time. Population statistics do not confirm this relationship, but these statistics may not tell the entire story because changes in the underlying risk of the population may have obscured the effect of prenatal care on overall rates of low birth weight. International comparisons also raise questions about the ability of prenatal care to reduce rates of low birth weight. American women are more likely than either German or Japanese women to start prenatal care in the first trimester, but rates of low birth weight and infant mortality are higher in the United States than they are in either Germany or Japan.

Alexander and Korenbrot also note that recent analyses indicate little is done during the standard prenatal care visits that could be expected to reduce low birth weight. For prenatal care programs to be effective in reducing low birth weight, these programs must target the known causes of low birth weight. Instead, most prenatal care programs are designed to detect major complications of pregnancy, not to prevent low birth weight. These programs start too late to address the problem of low prepregnancy weight, and few of them include intensive efforts to encourage pregnant women to stop smoking. New analyses also conclude that the often-cited economic benefits of prenatal care, which indicated that, for each dollar spent on prenatal care, direct medical costs savings of up to $3.38 would be realized, may be overstated.¹¹ In these calculations, the benefits were derived from the expected reductions in hospital stays and intensive care required for low birth weight infants. However, these benefits do not accrue if prenatal care does not significantly prevent low birth weight births. One innovative effort to provide an enhanced form of prenatal care¹² and other efforts to provide intensive interventions to prevent the onset of preterm labor have largely failed to demonstrate reductions in low birth weight.¹³

Prenatal care is a package of necessary services, and a number of benefits accrue from the receipt of these services which do not relate to the prevention of low birth weight. Prenatal care is often an adult woman's first contact with the medical care system, and the screening she receives may uncover manageable conditions and/or treatable diseases which could affect both the baby's life and her own. The education a woman receives about the pregnancy, labor and delivery, and caring for the newborn is very important, particularly for first-time mothers. Prenatal care is also valuable for women who are impoverished because it links them with other valuable social services. The legacy of prenatal care continues after the birth of a child because women who receive this care are more likely to get preventive care for their infants.¹⁴ Moreover, prenatal care may have other positive outcomes which are underassessed. All of these outcomes and others that are as yet unmeasured have potential economic benefits. Thus, the previously cited $3.38 savings for each dollar spent on prenatal care needs to be reassessed.

Social Programs

Hughes and Simpson discuss how the strong association between socioeconomic status and health problems in children (for example, low birth weight, infectious diseases, asthma, failure to thrive, teenage pregnancy, and child abuse) has resulted in the development of social programs aimed at reducing poverty and its devastating effects. While it is not clear how the mother's socioeconomic status translates into the birth of a low birth weight infant, it is thought that poverty with its associated reduced access to health care, poor nutrition, lower educational levels, inadequate housing, greater physical and psychological stress, and fewer life satisfactions may be responsible for some of the increased risk for low birth weight. In the United States, these programs have focused mainly on improving access to health and medical care for pregnant women, and on temporarily reducing the effects of poverty. However, it is not clear if these programs have had an important impact on low birth weight. Certainly these social programs were developed to ease the effects of poverty on children; in this they succeed. They should not be judged by their uncertain impact on low birth weight.

Hughes and Simpson describe how the introduction of the Medicaid program in 1965 and subsequent eligibility expansions in the 1980s brought federally funded prenatal and pediatric care to millions of impoverished pregnant women and their children. Reductions in low birth weight were found for women who were newly covered by the Medicaid program, but later expansions of eligibility to less severely socioeconomically deprived women were not associated with decreased rates of low birth weight. However, it is not clear how the provision of health care insurance even to women at highest risk translates into reductions in low birth weight, particularly since prenatal care appears to have only a small effect on reducing rates of low birth weight.

Other federal programs aimed at reducing the effects of poverty have also been implemented. The Special Supplemental Food Program for Women, Infants and Children (WIC) provides pregnant women with food vouchers, nutritional education, and referrals to other health and social services. The effects of the WIC program on reducing low birth weight births have been extensively evaluated, and the results have been mixed. Income supports for impoverished pregnant women became available with the start of the Aid to Families with Dependent Children (AFDC) program in 1935. The AFDC program, more commonly called welfare, is not nearly as well studied as the WIC program. There is some indication that the receipt of welfare is helpful in improving maternal weight gain during pregnancy, but there is little direct evidence to show any relationship between income supports obtained as part of welfare and reductions in low birth weight or preterm births.

Government-sponsored social programs for impoverished families were not designed to reduce rates of low birth weight or infant mortality, but to reduce the financial barriers to medical care for pregnant women and children, and some of the devastating effects of poverty. Therefore, it is not surprising to find that solutions designed to eliminate extreme poverty have made little impact on reducing national rates of low birth weight. The key to preventing this major public health problem does not appear to lie solely in socioeconomic solutions. However, finding ways to prevent low birth weight will probably require discovering reasons for the large variations in birth outcomes by socioeconomic status.

Lifestyle

There are lifestyle choices a woman can make to increase her chances of having a healthy normal birth weight child. Chomitz, Cheung, and Lieberman discuss the role that individual behaviors such as cigarette smoking, use of other drugs, and diet play in determining fetal growth. While women do make their own choices with regard to individual behaviors and overall lifestyles, they also face systemic, psychosocial, biological, and attitudinal barriers to lifestyle changes. In addition, having a healthful lifestyle may not be a high priority for many women who are more concerned with day-to-day survival. Women who are surrounded by poverty and violence and go without adequate housing, food, or employment may turn to unhealthful lifestyle choices as a means of coping. Chomitz and colleagues emphasize that expecting women simply to change their behavior without support and attention from the health care system, society, and influential people in their lives is unrealistic and fosters the belief that women are solely to blame for their undesirable behaviors.

While there are several things women can do to reduce their chances of having a low birth weight infant, one action stands out above all others—stop smoking cigarettes. Cigarette smoking is the single largest modifiable risk factor for low birth weight and infant mortality. It accounts for up to 20% of all low birth weight. Smoking retards fetal growth. On average, babies born to smokers weigh about one-half pound less than babies born to nonsmokers. Women who quit smoking during pregnancy significantly reduce the chances that their baby will be born low birth weight. However, most of these "preventable" low birth weight births would be concentrated among those infants born at or near term, who are moderately low birth weight, whose prognoses are quite good, and whose cost of medical care is moderate. Smoking is also associated with preterm birth, but this association is not nearly as strong as that for low birth weight.

Scientists have known about the consequences of smoking during pregnancy for more than 35 years, but they have made little progress in disseminating effective interventions to help pregnant women quit smoking or to keep young women from becoming addicted. Most health care providers are not trained to offer the specialized intensive services needed to help women stop smoking; most insurers do not cover the cost of smoking cessation interventions; and the programs that do exist do not focus on the smoker who is pregnant. Smoking is not a casual activity that can be easily stopped at will; it is a powerful addiction. The addictive powers of nicotine are illustrated in the results of a survey of drug addicts who reported that quitting cigarettes was much more difficult than quitting illicit drugs and alcohol.¹⁵

Other behaviors, such as diet and the abuse of alcohol and other drugs, while important, do not have nearly the impact on overall rates of low birth weight that cigarette smoking has. Abuse of alcohol and other drugs is associated with low birth weight and preterm birth, but relatively few pregnant women engage in drug abuse. The recent "epidemic" of cocaine use in the United States did not have a large effect on overall rates of low birth weight or preterm birth and may have been confined to local areas.¹⁶ However, some individual mothers and infants do suffer from the effects of drug abuse. Many women who desire to enter drug treatment programs are turned away because programs for drug-abusing pregnant women are generally unavailable.¹⁷

The situation is similar for diet. Because very few women in the United States are severely undernourished, only a small subset of women might be helped by dietary interventions. In addition, little impact on overall rates of low birth weight can be expected by reductions in the number of women having inadequate weight gain during pregnancy, low prepregnancy weight, and abuse of alcohol and other drugs during pregnancy. However, individual women can benefit greatly by abstaining from using drugs during pregnancy and by paying careful attention to their nutritional needs before and during pregnancy. Prenatal care can play an important role in assisting women to maintain healthful lifestyles.