Preterm Birth—Social Implications*
As discussed in Chapter 4, a recurrent theme in the Roundtable discussions has been the role of an expanded definition of environment, including the social environment, and its impact on human health. During the workshop, participants discussed the evidence suggesting an impact of the social environment on pregnancy outcome. Janet Rich-Edwards, Harvard Medical School, suggested that whether evaluating a social or environmental toxicant, it is important to look at the timing and duration of the exposure. In addition, she suggested, it is worth investigating factors that might have been present before the pregnancy was initiated to try to identify social predictors of preterm birth, to explore the concept of women becoming weathered or worn down by stress, to assess whether these social predictors endure over time, and to differentiate between the effects of chronic and acute stress.
Whether evaluating a social or environmental toxicant, it is important to look at the timing and duration of the exposure.
As more researchers and clinicians recognize the impact of stress resulting from an unhealthy environment on human health, stress has become the subject of intense research in the last decade. Previous research has established that low levels of stress activate cellular pathways that are necessary for growth, development, and maintenance of brain and muscle activity, as well as learning and memory. However, prolonged exposure to a chronic stressor or exposure to an acute stressor that overwhelms the individual’s ability to return to homeostasis can result in ill health. The individual’s ability to handle stress depends not only on the strength of the agent but also on the host’s susceptibility to stress and on the social environment that surrounds the individual. Social environment factors such as education, family status, household income, race and ethnicity, social support, coping, and repressive coping style, as well as violence, can predetermine the outcome of pregnancy. The individual’s degree of social stress is related to the stability of the pre- and postnatal social environment in which it lives, the amount of social support which the individual receives from its peers and family, and its social experiences during behavioral development.
Two kinds of stressors have been most extensively studied with respect to their impact on pregnancy outcomes: acute stressful life events and hard physical work—a type of individual chronic exposure. A third kind of stressor, individual experiences of racism, according to some participants, may explain at least part of the excess preterm delivery rate among African and Mexican Americans who have lived in the United States for most or all of their lives. However, said Fernando Guerra, San Antonio Metropolitan Health District, some of the conditions revealed in the data about the health of Mexican Americans, related to nutrition, diet, life-style, risk-taking behavior, domestic violence, displacement from the security of family and home, conditions of overcrowding, and lack of attachment to strong religious ties, indicate that these are stressors that must be considered and studied further.
Epidemiological Model of Stress
Carol Rowland Hogue of Emory University suggested that the classic “host, environment, agent” triangle of epidemiological causality can illustrate what might be included in a more comprehensive test of the stress–preterm delivery hypothesis. In this model, the host is the individual woman, the environment is her social and cultural context, and the agent is the immediate stressful event(s) requiring her response. This framework provides a context to determine the extent to which a given theory may include important and potentially interrelated factors.
In considering host susceptibility, it is important to note that individuals may differ in psychological and physiological responses to the same stressor,
reflecting differences in the context within which the stressor occurs and differences in individual response to that type of stressor.
Research Application of the Model
Three studies have found that when pregnant women express anxiety about their pregnancy, they are more likely to experience a preterm delivery. These studies collected information on pregnancy anxiety prior to delivery; however, it is not possible to ascertain whether subtle pregnancy complications, which might have alerted the women to impending problems, had triggered the pregnancy anxiety and, therefore, were also the cause of the preterm delivery, according to Hogue.
In one small study among pregnant women, elevated blood pressure—in response to an arithmetic test—was associated with both decreased birth weight and gestational age. In experimental studies of African-American and Caucasian participants, blood pressure following cardiovascular activity, at rest, and after a stressful stimulus was greater in African Americans. This increased cardiovascular reactivity among African Americans does not appear to be associated with familial history of hypertension, suggesting that individuals’ exposure history, rather than genetic differences, may explain differences in host susceptibility to stressors. Early and continuous stressors may also cause learned physiological responses that trigger higher reactivity when similar stress occurs later in life. For example, early and continual experiences of individual racism have been hypothesized to sensitize African-American women to stress reactivity, said Hogue.
Previous and Acute Stress in the Household
The issue of women being weathered or worn down by stress, also known as weathering, suggests that the environment is literally incorporated into the host. Environment may weather a woman and increase her susceptibility to giving birth to a premature baby through the altered hormonal responses to stress, according to Rich-Edwards. For example, repeated exposure to threats during childhood, such as may be encountered in a violent household, may repeatedly activate the hypothalamic–pituitary–adrenal (HPA) axis response to stress and actually reshape that axis. Entering adulthood with an altered HPA axis, a female may find herself with a hair-trigger response to stress, or “kindling” or “arousal pathology.”
Violence and chronic stress in the household can potentially have profound implications on reproductive health, according to Rich-Edwards. One study in Glasgow, Scotland, showed that chronic strain in the household was associated with both preterm delivery and low birth weight. Further, data showed that mean
peak Adrenocorticotropic hormone (ACTH) response to laboratory stressors is higher among women who were abused as children than among those who were not (Figure. 5.1).
Early abuse was associated with an exaggerated ACTH response, whether or not the women experienced depression afterwards. Differences were also observed in cortisol response and heart rate response among women who were both abused and depressed.
Rich-Edwards believes that the grim statistics surrounding violence may potentially have profound implications for stress response among women. A nationally representative phone survey of 8,000 women ages 18 or older conducted from 1995 to 1996 by the National Institute of Justice and the Centers for Disease Control and Prevention asked women about their lifetime exposures to violence. The survey found that one out of every two women had been physically assaulted at some point in her life, either as a child by an adult caretaker or as an adult (Tjaden et al., 2000). In addition, one out of every six women experienced rape or attempted rape. Data on the rate of intimate partner violence to women showed a marked association between low income and rate of abuse.
Of those who experienced rape, 22 percent were raped before age 12 and another 32 percent were raped between the ages of 12 and 17, indicating that the majority of these victims were raped as children. According to some participants, we need to understand further the role of the various types and levels of stress on premature birth.
Another consideration is the environment or the social and cultural context within which a woman lives. Ameliorating environmental factors include intimate social support, which has been associated with improved pregnancy outcomes in a number of studies. However, attempts to replicate social support in clinical trials to prevent poor pregnancy outcomes have not been effective. The chronic stress associated with environmental or social stressors may be measured at the individual level, for example, through inquiring about the level of daily difficulties experienced as well as about perceptions of environmental issues. However, the negative health impact of environmental stressors depends not only on individual perception, but also on actual environmental risks. For this reason, measurement of environmental risks must include contextual variables. Types of stressful environmental factors include those associated with gender, socioeconomic status, and race or ethnicity.
Several years ago, a series of studies found that maternal stress during pregnancy was associated with poor pregnancy outcomes, including preterm delivery, but only for women who had fewer social supports available to buffer stressful events. Subsequent studies of stress, social support, or combinations of stress and social support conducted in many settings have produced only a few significant findings and usually with small odds ratios. Interventions aimed at increasing social support have, in general, not lengthened gestation, said Hogue. The lack of success in these trials could be due to problems in design or analysis, or the trials may have been diluted by including women in the intervention group who did not need social support. If the presence of stressful events combined with the lack of social support during pregnancy does increase the risk of preterm delivery, then randomized trials that increase social support for women in need of such support should serve to lengthen gestation.
Interventions aimed at increasing social support have, in general, not lengthened gestation.
Carol Rowland Hogue
The association between poverty and poor pregnancy outcomes has been found consistently across populations over time and by various measures of social class and social status. Causal models explaining this association point to the greater exposure to negative life events experienced by poor individuals. Additionally these individuals have fewer coping resources available for adapting to stress, and traditionally, they live within a culture of poverty that encourages hopelessness and resorting to unhealthy coping strategies. Also, poor communities are more exposed to environmental hazards such as lead, toxic agents,
and unsafe neighborhoods, and they do not have adequate community resources for responding to health and medical emergencies. Including these communitylevel factors in studies of stress and preterm delivery requires group-level measurements of exposure. Such factors have not been utilized widely in epidemiological research in these areas, but a few studies are beginning to confirm the utility of pursuing these contextual causes.
Social position, Rich-Edwards noted, is an example of a strong predictor of preterm birth, which has implications for the timing of chronic stress. Women’s education level is a predictor of low birth weight births across racial and age lines. In all categories, however, African-American women are more likely to have low birth weight babies than Caucasian women. For example, a college-educated African-American mother still has twice the risk of having a low birth weight baby compared to a Caucasian college-educated mother. Further, among African American infants, mortality is lower in cities with less residential segregation (an example of institutional racism), a finding that is independent of the effect of poverty on infant mortality. In addition to the growing evidence that institutionalized and interpersonal racism adversely affects the health of African-American infants, racism in the United States may also affect the reproductive health of Mexican Americans.
Women’s education level is a predictor of low birth weight births across racial and age lines.
Coping strategies, in contrast to coping resources, describe specific behavioral or cognitive attempts to manage stressful demands. Problem-focused coping is more likely to be used when the stressor is perceived as controllable or modifiable, while emotion-focused coping is used more frequently when the stressor is perceived as uncontrollable. To date, research results are inconclusive regarding whether either strategy is more effective in buffering the effects of stressors, according to Hogue. Another important buffer may be spirituality, or reliance on a force beyond the individual. This too has yet to be explored with respect to preterm delivery. Among the most common coping strategies known to have deleterious effects on the outcome of pregnancy are alcohol use, drug dependence, and cigarette smoking.