A relatively small number of studies have assessed the effects of daily stressors on preterm birth but have shown disappointing results (Paarlberg et al., 1996; Wadhwa et al., 1993). It is possible that these measures do not capture levels of stress exposure high enough to influence pathways to prematurity. Although daily stressors may operate in combination with other stress exposures, such as major life events, and interact with responses such as anxiety or depression to contribute to the risk of preterm birth, they do not seem promising overall for predictive purposes.
Two other bodies of research are relevant to the role of stress and preterm delivery. One is on the effects of occupational or work stress on preterm delivery (Woo, 1997). This area is related to but distinct from the topics of physical activity and employment discussed earlier in this chapter. Whether or not a woman is employed, she may engage in various degrees of daily physical activity or strain, but only women who are employed would potentially experience occupational stress. Savitz and Pastore (1999) highlighted previous studies on occupational stress and preterm birth that were consistent in showing significant associations of either occupational stress or physical strain with preterm labor or delivery (Brandt and Nielsen, 1992; Brett et al., 1997; Henricksen et al., 1994; Homer et al., 1990). In a related study, Pritchard and Teo (1994) found that assessments of household strain were significant predictors of preterm birth in 393 Swedish women.
This area is complex because it has sometimes been framed as an issue of whether employment during pregnancy is itself risky or whether the characteristics of a woman's employment are the preterm birth risk-associated factor. Key issues include the type of work activities, the number of hours of work each day or week, the time (trimester) during pregnancy when a woman works, the work environment, and the psychological strain (cognitive and emotional) that may be associated with work. When these factors are quantified, it may be found that some employed women are at risk and others are not. This is another topic worthy of future investigation.
A second area relevant to stress is that on exposure to personal violence. A small but growing body of work suggests that women who experience domestic or personal violence during pregnancy are at risk for adverse birth outcomes (Amaro et al., 1990; Coker et al., 2004; Parker et al., 1994; Rich-Edwards et al., 2001; Shumway et al., 1999). The extent to which this is the result of stress processes rather than other mediating processes is unclear, however. In addition, most studies appear to view domestic or personal violence as a chronic stressor and have observed that violence affects birth weight but not preterm birth.
Mechanisms Linking Stress and Emotions to Preterm Birth
Maternal stress can cause the release of increased levels of catecholamines and cortisol, which could prematurely activate placental corticotropin-releasing hormone, thereby precipitating the biological cascade leading to the onset of preterm labor (see Chapter 6). Stress can also alter immune function, leading to increased susceptibility to intra-amniotic infection or inflammation (Wadhwa et al., 2001). Additionally, stress may induce high-risk behaviors as a means of coping with stress (Whitehead et al., 2003).
Evidence is also accumulating that infections may play a key role in the pathogenesis of preterm birth, particularly very preterm delivery (see Chapter 6). Although researchers have re cently focused on BV, several other infections, including asymptomatic bacteriuria, sexually transmitted infections, and peridontal infections, have all been implicated.
There is a need for investigation of the specific pathways whereby distinct stress and emotional and affective factors contribute to preterm birth. Past research provides some clues to possible avenues of investigation. For example, maternal anxiety has been implicated more in early labor and delivery via HPA pathways, whereas depression has been associated with poor health behaviors and their consequences for fetal growth. In particular, greater theoretical analysis of the intensity and duration of distinct emotional states such as anxiety and depression and their consequences for pregnancy outcomes, such as spontaneous preterm labor, spontaneous rupture of membranes, and fetal growth restriction, is needed. (See Chapter 6 for a more extensive review and discussion of the pathways from stress to preterm birth.)
More specifically, the role of anxiety in the preterm pathogenesis process has been underdeveloped (Kurki et al., 2000; McCool et al., 1994). One notion, built on common anecdotes, is that a single episode of strong emotion, such as anxiety from being in New York City when the World Trade Center was attacked, in New Orleans during Hurricane Katrina, or in Los Angeles during the Northridge Earthquake, can precipitate early labor. A second possibility is that a chronic state of anxiety resulting from a clinically diagnosable anxiety disorder or subclinical set of symptoms places a woman at risk for preterm delivery. A third possibility is that a combination of the first two possibilities, in the form of an anxious disposition combined with a highly stressful acute event or series of events, may interact to cause early labor.
Limiting the inquiry to the role of anxiety and its biological consequences may prove more fruitful than earlier and cruder approaches to studying general distress and its influence on preterm delivery. Earlier, such studies served the field well in identifying potentially new risk factors, but more scientific precision on the emotional experience of pregnancy and its consequences is greatly needed now to obtain a further understanding of the association of stress with preterm delivery. For example, emerging research indicates that physiologic stress reactivity (e.g. endocrine and cardiovascular) decreases across gestation (de Weerth and Buitelaar; 2005; Glynn et al., 2001; Glynn et al., 2004; Matthews and Rodin, 1992; Schulte et al., 1990) which has substantive implications for the role of psychosocial factors as both risk factors and targets for intervention.
Culture, Race, Ethnicity, and Stress
A complication of research on stress in general and on emotional states during pregnancy more specifically is that emotional experience and responses to emotion are at least partially culturally grounded (Mesquita and Frijda, 1992). That is, people of different cultures differ in their comprehension of and ability to accept the expression of emotions, such as anxiety and sadness. In some subgroups in the United States, the expression of anxiety may be much more normative and accepted than in others, in which it may be frowned upon, misunderstood, or ignored. Languages may also differ in their abilities to translate the word "stress." The Spanish language, for example, does not have a specific translation for the word "stress." Anxiety, or nervios, is understood in Spanish, whereas stress in general is not. Thus, studies of stress as a risk factor by the use of standard scale assessments delivered in Spanish may or may not be assessing the same phenomenon that these scales assess when they are delivered in English. This poses a special challenge to researchers.
Similarly, African-American women, whose rates of preterm delivery and infant mortality are the highest in the United States, have unique experiences of stress, yet there is a dearth of studies on African-American cultural factors pertaining to stress, emotion, or pregnancy. Parker Dominguez and colleagues (2005) found that neither anxiety nor perceived stress was significantly correlated with gestational age or low birthweight among 179 pregnant African-American women. Instead, a newer measure of the extent to which women experienced intrusive thoughts or rumination about their two most severe major life events was associated with lower birth weight when gestational age in linear multiple regression analyses was controlled for. Intrusive thought is a recognized symptom of trauma containing both cognitive and emotional components (and is often symptomatic of posttraumatic stress disorder).
The possibility that low-income African-American women experience more symptoms of trauma and that these are more important risk factors for preterm birth than depression or anxiety for this or other groups is intriguing. More generally, researchers must address the possibility that the same aspects of stress may not pose a risk for preterm birth in the same manner for all racial and ethnic groups. In-depth studies of specific racial-ethnic and cultural groups that include culturally specific stress measures may yield answers to whether stress is a risk factor for preterm birth for specific groups. The answers may be more complex than has been imagined. Anxiety may be a stronger risk factor for Latinos and whites, whereas depression, posttraumatic stress disorder, or racial stressors may be more potent individual-level risk factors for African-American women. These possibilities might help to explain why research on stress and pregnancy outcomes has yielded equivocal findings and also why the findings from studies in foreign countries with more homogeneous populations, such as Denmark, have been more definitive. Furthermore, these possibilities suggest very different intervention strategies for different racial and ethnic groups (Norbeck and Anderson, 1989).
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