Race, Cardiovascular Reactivity, and Preterm Delivery Among Active-Duty Military Women.

Abstract


Background:

Rates of preterm delivery in the United States are higher in black women compared with whites. In this study, we examined cardiovascular reactivity and risk of preterm delivery among black and white military women.

Methods:

We recruited a total of 500 black and white active-duty military women from the prenatal clinic at a large military installation, interviewing them early in pregnancy and again at 28 weeks of gestation. A subgroup of women underwent a computerized stress test to determine cardiovascular reactivity assessed as increases in heart rate and blood pressure compared with measurements taken before the stress test.

Results:

Despite a relatively low overall risk of preterm delivery (8.2%), we found the same 2-fold racial disparity reported in other populations (hazard ratio for preterm delivery in black women vs whites = 2.30). The disparity is present in all military ranks and is largest for medically indicated preterm deliveries. Among the 313 subjects who participated in the computerized stress testing, blacks exhibited more cardiac reactivity than whites. In black subjects only, a 1-mm increase in diastolic blood pressure reactivity was associated with 1.1 a day earlier delivery (-0.17 weeks). A similar trend was seen with heart rate.

Conclusions:

Autonomic dysfunction after exposure to stressors may play a role in the timing of delivery among black women.

Introduction

The disparity in rates of preterm delivery between black and white women in the United States is striking, seemingly intractable and not easily explained. Many of the suspected social and biologic differences between black and white women have been studied but seem to account for very little of the difference in rates of preterm delivery. Among the explanations that have been proposed are that black women are subjected to more stressors and are more susceptible to these stressors at the physiological level.

One indicator of susceptibility to stressors is cardiovascular reactivity, which is defined as the propensity for an individual to undergo changes in blood pressure and heart rate during exposure to a stressor. Higher levels of cardiovascular reactivity have been reported among black men and women. Hemodynamic reactivity is presumed to be mediated by the neuroendocrine system, in particular norepinephrine. In theory, alterations in maternal cardiovascular functioning coincident with changes in endocrine activity could lead to adverse pregnancy outcome. Also, autonomic dysregulation may be associated with pregnancy complications that increase risk of preterm delivery.

Two recent studies have examined the influence of cardiovascular reactivity on gestational age at delivery. McCubbin and colleagues in Kentucky found that in a sample of 40 primigravid women from a university prenatal clinic, diastolic blood pressure reactivity to a cognitive challenge (an interactive arithmetic task) was inversely related to both gestational age and birth weight; Pearson correlation coefficients were r = -0.44 and -0.39, respectively. Each millimeter of mercury increase in diastolic blood pressure was associated with a 0.31-week decrease in gestation at delivery. Systolic blood pressure and heart rate showed more modest correlations in the same direction. A study of 70 healthy pregnant women in Argentina found that vascular reactivity (specifically, diastolic blood pressure increase) as assessed by the cold pressor test (immersing the subject's hand in cold water) also showed a negative correlation with gestational age (r = -0.57). Each mean millimeter of mercury increase in reactivity was associated with a 0.07-week decrease in gestational age at delivery.

We examined cardiovascular reactivity measured after mental challenge and the risk of preterm delivery in black military women as compared with their white counterparts.

DISCUSSION

Despite a relatively low overall risk of preterm delivery (8.2%), the risk of white (6.2%) and black (13.9%) active-duty military women from the same Air Force facility showed a more than 2-fold difference. The only previous study of racial disparity in pregnant military women, by Adams et al, also found an excess risk of preterm delivery for black women relative to white women, with the association being stronger for medically indicated preterm deliveries and for deliveries before 33 weeks, as in our study.

Blacks have been found to have greater reactivity to stressors than whites, particularly in the vasculature, and theory has predicted that this greater reactivity may underlie their increased risk for hypertension. Hyperreactivity might also be implicated in preterm delivery either directly or indirectly through hypertensive disorders of pregnancy.

Like the 2 previous studies of pregnant women in Kentucky and Argentina, we observed an association between increased reactivity and gestational age at delivery. In our study, the association was restricted to the subgroup of black women. The study in Kentucky found a marginally significant interaction between diastolic reactivity and maternal race with the results suggesting a stronger association in blacks. As in both of the previous studies, the association we found was primarily with diastolic blood pressure. The observed decrease of 0.17 weeks’ gestation with a 1-mm Hg increase in DBP is less than the 0.31 weeks observed by McCubbin and more than the 0.07 weeks reported by the Argentinian group.

In our data, the association of blood pressure reactivity with gestational age at delivery, although modest, is twice as large among all deliveries as among spontaneous deliveries. This is consistent with reactivity being a mediating mechanism for certain pregnancy complications that lead to medical intervention.

Our study has limitations that should be considered. The results for reactivity are based on the approximately two thirds of subjects who participated in the psychophysiological testing; however, this is a participation rate not unlike that for the onerous component of many other studies. The nontest takers had greater exposure to sources of job stress and a higher rate of preterm delivery. This loss reduced sample size and might have introduced a selection bias. Although we observed an association between reactivity and gestational age among black women, we cannot evaluate whether this reactivity may have contributed to the black/white disparity in risk of preterm delivery. This is because in the stress test group, the percent delivering preterm among the black women was fairly similar to that among the whites, reflecting the higher risk of preterm delivery in the nontest takers.

In addition, we were unable to ascertain pregnancy outcomes for 63 subjects who left the military base before delivery. It is difficult to imagine that this would be a biased subset, however, because their relocation was determined by military considerations. Finally, an underlying assumption is that reactivity to laboratory stressors correlates with reactivity to real life stressors, a point on which there is little evidence, although the best designed studies are supportive.

In conclusion, we observed a 2-fold racial difference in preterm delivery in a population of active-duty military women, a difference that was present within groups stratified by rank and a difference we were unable to explain. Nevertheless, our results with respect to cardiovascular reactivity to stressors suggest a promising direction. Reactivity levels in our study were higher among black women than whites and, in the subgroup of black women, reactivity in diastolic blood pressure showed an inverse correlation with gestational age at delivery. Our data suggest that among black women, cardiac reactivity may contribute to the risk of earlier delivery