Race, cardiovascular reactivity, and preterm delivery among active-duty military women

Integrative Review of Racial Disparities in Perinatal Outcomes Among Beneficiaries of the Military Health System

OBJECTIVE

To examine the extent to which racial disparities exist in the perinatal outcomes of beneficiaries of the Military Health System (MHS).

DATA SOURCES

We searched the PubMed, CINAHL, and Embase databases.

STUDY SELECTION

We selected articles published in English in peer-reviewed journals in which the authors examined race in relation to perinatal outcomes among beneficiaries of the MHS. Date of publication was unrestricted through March 2021.

DATA EXTRACTION

Twenty-six articles met the inclusion criteria. We extracted data about study design, purpose, sample, setting, and results. We also assigned quality appraisal ratings to each article.

DATA SYNTHESIS

In most of the included articles, researchers observed differences in perinatal outcomes between Black and White women. Compared to White women, Black women had greater rates of cesarean birth, preterm birth, low birth weight, and small for gestational age neonates. White women had greater rates of postpartum depression than Black women.

CONCLUSION

Racial disparities in very low birth weight newborns and preterm birth may be smaller in the MHS than in the general population of the United States. The overall rates of preterm birth, cesarean birth, and neonatal mortality were lower for beneficiaries of the MHS than in the general population of the United States.

A Scoping Review of Pregnancy, Childbirth, and the Postpartum Period in Active Duty U.S. Military Women

PURPOSE

Women in the U.S. military encounter unique challenges during the perinatal period that are driven by military requirements for mission readiness. The purpose of this scoping review was to systematically examine the extent, range, and nature of the literature on pregnancy, childbirth, and the postpartum period of active duty military women. A secondary aim was to identify leverage points for changes to improve perinatal health of servicewomen.

METHODS

We used a PRISMA-ScR protocol to guide this scoping review of research and non-research articles germane to the perinatal health of servicewomen. In the protocol, we identified the rationale, objectives, eligibility criteria, search strategy, sources of evidence, and data charting processes for the review. We used the social ecological model for military women's health framework to guide the synthesis of results.

FINDINGS

Eighty-four articles on the topics of pregnancy, childbirth, and the postpartum period of servicewomen published from 2000 to 2018 were identified. The articles were mainly research studies (n = 76), of which 49 had observational designs. Leverage points to promote workplace safety and support of pregnant women, perinatal screening, recognition of pregnancy and postpartum depression, and maintaining physical fitness during pregnancy and the postpartum period were identified in multiple levels of the social ecological model for military women's health.

CONCLUSIONS

Literature published from 2000 to 2018 is broad in scope, yet generally lacks a robust body of evidence on any one topic. Implementing strategies and military policies that are directed at the identified leverage points could enhance the health of childbearing servicewomen.

Racial Disparities in Prenatal Care Utilization and Infant Small for Gestational Age Among Active Duty US Military Women

OBJECTIVES

To examine racial disparities in prenatal care (PNC) utilization and infant small for gestational age (SGA) among active duty US military women, a population with equal access to health care and known socioeconomic status.

METHODS

Department of Defense Birth and Infant Health Research program data identified active duty women with singleton live births from January 2003 through August 2015. Administrative claims data were used to define PNC utilization and infant SGA, and log-binomial regression models estimated associations with race/ethnicity. To examine whether associations between maternal race/ethnicity and infant SGA were subject to effect measure modification, respective analyses were stratified by demographic and health characteristics.

RESULTS

Overall, 12.2% of non-Hispanic White women initiated PNC after the first trimester, compared with 14.8% of American Indian/Alaska Native, 15.1% of Asian/Pacific Islander, 14.2% of non-Hispanic Black, and 13.0% of Hispanic women. Infant SGA prevalence was 2.4% and 1.6% among non-Hispanic Black and White women, respectively (aRR 1.52, 95% CI 1.40-1.64). This disparity persisted across stratified analyses, particularly among non-Hispanic Black versus White women with a preeclampsia or hypertension diagnosis in pregnancy (RR 1.96, 95% CI 1.67-2.29) and those aged 35 + years at infant birth (RR 2.04, 95% CI 1.56-2.67).

CONCLUSIONS FOR PRACTICE

In multiple assessments of PNC utilization and infant SGA, non-Hispanic Black military women had consistently worse outcomes than their non-Hispanic White counterparts. This suggests that equal access to health care does not eliminate racial disparities in outcomes or utilization; additional research is needed to elucidate the underlying etiology of these disparities.

Acknowledging and Addressing Allostatic Load in Pregnancy Care

The USA is one of the few countries in the world in which maternal and infant morbidity and mortality continue to increase, with the greatest disparities observed among non-Hispanic Black women and their infants. Traditional explanations for disparate outcomes, such as personal health behaviors, socioeconomic status, health literacy, and access to healthcare, do not sufficiently explain why non-Hispanic Black women continue to die at three to four times the rate of White women during pregnancy, childbirth, or postpartum. One theory gaining prominence to explain the magnitude of this disparity is allostatic load or the cumulative physiological effects of stress over the life course. People of color disproportionally experience social, structural, and environmental stressors that are frequently the product of historic and present-day racism. In this essay, we present the growing body of evidence implicating the role of elevated allostatic load in adverse pregnancy outcomes among women of color. We argue that there is a moral imperative to assign additional resources to reduce the effects of elevated allostatic load before, during, and after pregnancy to improve the health of women and their children.

Maternal Pre-Pregnancy Cardiovascular Risk Factors and Offspring and Grandoffspring Health: Bogalusa Daughters

Both maternal pre-pregnancy body mass index (BMI) and gestational weight gain have been associated with cardiovascular health in the offspring beyond two generations. A total of 274 daughters (aged 12⁻54) of 208 mothers who participated in the Bogalusa Heart Study were interviewed about their reproductive history. Mothers' data was taken from the original study, and cardiovascular measures at the visit prior to pregnancy were correlated with daughter's measures. Maternal pre-pregnancy BMI, skinfold, and waist circumference were examined as a predictor of daughters' blood pressure, lipids, and glucose, as well as a predictor of birthweight and gestational age of grandchildren. Maternal pre-pregnancy BMI was associated with higher blood pressure and lower low-density lipoprotein (LDL) and cholesterol in the daughters. Most maternal cardiometabolic risk factors were not associated with grandchildren's birth outcomes, even though higher cholesterol and LDL was associated with lower gestational age, and higher BMI and skinfold thickness with an increased risk of preterm birth. In this pilot study, some associations were found between maternal adiposity and cardiovascular risk, daughters' cardiovascular risk, and grandchild birth outcomes. Lack of conclusive associations could be due to a true lack of effect, effects being primarily mediated through daughter's BMI, or the low power of the study.

Lower Preterm Birth Rates but Persistent Racial Disparities in an Open-Access Health Care System

BACKGROUND

The Military Health System (MHS) provides universal access to medical care to active duty service members, retirees, and their dependents. Observational data from small studies suggest lower preterm birth rates in the MHS compared with U.S. national averages. The objectives of this study are to determine the rate of preterm birth in the MHS from 2006 to 2012 compared with national rates and to analyze the impact of demographic factors on preterm birth in a universal access health care system.

METHODS

A cohort of infants born in 2006-2012 was formed from the MHS M2 database. International Classification of Disease - Ninth Revision (ICD-9) codes were used to define prematurity. Preterm births were linked to military parent's demographic data. Calculated MHS preterm birth rates were compared with U.S. national data using the Pearson chi-square test and comparison via standardized differences. Logistic regression analyses were used to determine the impact of demographic factors on prematurity.

FINDINGS

From 2006 to 2012, 564,920 infants were born in the MHS; 45,445 (8%) were born preterm. The preterm birth rate in the MHS peaked at 8.34% in 2008 and declined to 7.67% in 2012, which is significantly lower than the U.S. national average preterm birth rate over the same time period. In the 2008-2012 cohort, the odds of preterm birth were increased with Black race (adjusted odds ratio 1.30; 95% confidence interval 1.26-1.33) and with a parent of junior enlisted rank (adjusted odds ratio 1.08; 95% confidence interval 1.05-1.06), a surrogate for lower socioeconomic status. Odds of preterm birth were decreased in families with married parents and with an active duty mother.

DISCUSSION

Preterm birth rates in the MHS have been consistently lower than national rates from 2006 to 2012, potentially due to universal access to health care. Black race increased odds of preterm birth despite universal access to health care. These findings support the need for further research examining racial disparities in health care outcomes related to preterm birth.

Pre-pregnancy cardiovascular risk factors and racial disparities in birth outcomes: the Bogalusa Heart Study

BACKGROUND

Racial disparities in birth outcomes are mirrored in cardiovascular health. Recently there have been calls for more attention to preconception and interconceptional health in order to improve birth outcomes, including as a strategy to reduce black-white disparities.

METHODS

As part of a larger study of cardiovascular and reproductive health ("Bogalusa Babies"), female participants were linked to their children's birth certificates for Louisiana, Mississippi, and Texas births from 1982 to 2009. Three thousand and ninety-five women were linked to birth certificate data. Birth outcomes were defined as low birthweight (LBW) birthweight < 2500 g; preterm birth (PTB), > 3 weeks early; small for gestational age (SGA), <10th percentile for gestational age (percentiles based on study population); large for gestational age (LGA) >90th percentile for gestational age]. Cardiovascular measures (blood pressure, lipids, glucose, insulin) at the visit closest in time but prior to the pregnancy was examined as predictors of birth outcomes using logistic models adjusted for covariates.

RESULTS

Only a few cardiovascular risk factors were associated with birth outcomes. Triglycerides were associated with higher risk of LBW among whites (aOR 1.05, 95% 1.01-1.10). Higher glucose was associated with a reduction in risk of SGA for black women (aOR 0.85, 95% CI 0.76-0.95), but not whites (p for interaction = 0.02). Clear racial disparities were found, but they were reduced modestly (LBW/SGA) or not at all (PTB/LGA) after CVD risk factors were adjusted for.

CONCLUSIONS

This analysis does not provide evidence for preconception cardiovascular risk being a strong contributor to racial disparities.

Cervical pathways for racial disparities in preterm births: the Preterm Prediction Study

Purpose: Racial disparities in preterm birth have been long recognized, but the social and biological mechanisms for these differences are unclear. Our analysis had three goals: (1) to determine the relation between race and other social risk factors and cervical structure; (2) to determine whether social factors mediate the relation between race and cervical structure; and (3) to determine whether racial disparities in preterm birth (PTB) are mediated through changes in cervical structure observed earlier in pregnancy. Materials and methods: Data from the Maternal Fetal Medicine Unit network Preterm Prediction Study were used to examine the relation between race and other social factors and cervical properties throughout pregnancy in 2920 black and white women. Outcomes included cervical length and dilation; cervical score (cervical length-internal dilation); and whether membranes protruded at 22-24 and 26-29 weeks. Race, education, income, insurance type, and marital status were examined as predictors of the outcomes using linear and logistic regression, adjusting for age, BMI, parity, and smoking. Mediation analysis was used to examine whether (a) any social factors explained racial differences in cervical properties, and (b) whether cervical properties mediated racial differences in risk for preterm birth. Results: Shorter cervical length, especially at a subject's first visit, was associated with black race (adjusted beta -1.56 mm, p < .01) and lower income (adjusted beta -1.48, p =.05). External dilation was not associated with social factors, while internal dilation was associated with black race and lower education. Black race and marital status were associated with lower cervical score. There was no evidence of mediation of the racial effect on cervical properties by any social factor. Shorter cervical length, dilation, and score were all associated with preterm birth (p < .01). Mediation analysis indicated that each of these mediated the effect of race, but explained a small proportion of the total effect (15-25%). Conclusions: Race, and, to a lesser extent, other social factors are correlated with adverse cervical properties. This pathway could explain a proportion of the racial disparity in preterm birth.

Perinatal distress in women in low- and middle-income countries: allostatic load as a framework to examine the effect of perinatal distress on preterm birth and infant health

In low- and middle-income countries (LMIC), determinants of women’s and children’s health are complex and differential vulnerability may exist to risk factors of perinatal distress and preterm birth. We examined the contribution of maternal perinatal distress on preterm birth and infant health in terms of infant survival and mother–infant interaction. A critical narrative and interpretive literature review was conducted. Peer-reviewed electronic databases (MEDLINE, Embase, Global Health, CINHAL), grey literature, and reference lists were searched, followed by a consultation exercise. The literature was predominantly from high-income countries. We identify determinants of perinatal distress and explicate changes in the hypothalamic–pituitary–adrenal axis, sympathetic, immune and cardiovascular systems, and behavioral responses resulting in pathophysiological effects. We suggest cultural–neutral composite measures of allostatic mediators (i.e., several biomarkers) of maternal perinatal distress as objective indicators of dysregulation in body systems in pregnant women in LMIC. Understanding causal links of maternal perinatal distress to preterm birth in women in LMIC should be a priority. The roles of allostasis and allostatic load are considered within the context of the health of pregnant women and fetuses/newborns in LMIC with emphasis on identifying objective indicators of the level of perinatal distress and protective factors or processes contributing to resilience while facing toxic stress. We propose a prospective study design with multiple measures across pregnancy and postpartum requiring complex statistical modeling. Building research capacity through partnering researchers in high-income countries and LMIC and reflecting on unique ethical challenges will be important to generating new knowledge in LMIC.

Psychobiobehavioral Model for Preterm Birth in Pregnant Women in Low- and Middle-Income Countries

Preterm birth (PTB) is a final common outcome resulting from many interrelated etiological pathways; of particular interest is antenatal psychosocial distress (i.e., stress, anxiety, and depression). In LMI countries, both exposure to severe life stressors and rate of PTB are on average greater when compared with high-income countries. In LMI countries women are exposed to some of the most extreme psychosocial stress worldwide (e.g., absolute poverty, limited social resources). High prevalence of antenatal stress and depression have been observed in some studies from LMI countries. We propose a psychosocial, biological, and behavioral model for investigating the complex multisystem interactions in stress responses leading to PTB and explain the basis of this approach. We discuss ethical considerations for a psychosocial, biological, and behavioral screening tool to predict PTB from a LMI country perspective.

Stress-induced inflammatory responses in women: effects of race and pregnancy

OBJECTIVE

African Americans experience preterm birth at nearly twice the rate of whites. Chronic stress associated with minority status is implicated in this disparity. Inflammation is a key biological pathway by which stress may affect birth outcomes. This study examined the effects of race and pregnancy on stress-induced inflammatory responses.

METHODS

Thirty-nine women in the second trimester of pregnancy (19 African American, 20 white) and 39 demographically similar nonpregnant women completed an acute stressor (Trier Social Stress Test). Psychosocial characteristics, health behaviors, and affective responses were assessed. Serum interleukin (IL)-6 was measured at baseline, 45 minutes, and 120 minutes poststressor.

RESULTS

IL-6 responses at 120 minutes poststressor were 46% higher in African Americans versus whites (95% confidence interval = 8%-81%, t(72) = 3.51, p = .001). This effect was present in pregnancy and nonpregnancy. IL-6 responses at 120 minutes poststressor tended to be lower (15%) in pregnant versus nonpregnant women (95% confidence interval = -5%-32%, p = .14). Racial differences in inflammatory responses were not accounted for by demographics, psychological characteristics, health behaviors, or differences in salivary cortisol. Pregnant whites showed lower negative affective responses than did nonpregnant women of either race (p values ≤ .007).

CONCLUSIONS

This study provides novel evidence that stress-induced inflammatory responses are more robust among African American women versus whites during pregnancy and nonpregnancy. The ultimate impact of stress on health is a function of stressor exposure and physiological responses. Individual differences in stress-induced inflammatory responses represent a clear target for continued research efforts in racial disparities in health during pregnancy and nonpregnancy.

Physiological reactivity to psychological stress in human pregnancy: current knowledge and future directions

Cardiovascular and neuroendocrine reactivity to acute stress are important predictors of health outcomes in non-pregnant populations. Greater magnitude and duration of physiological responses have been associated with increased risk of hypertensive disorders and diabetes, greater susceptibility to infectious illnesses, suppression of cell-mediated immunity as well as risk for depression and anxiety disorders. Stress reactivity during pregnancy has unique implications for maternal health, birth outcomes, and fetal development. However, as compared to the larger literature, our understanding of the predictors and consequences of exaggerated stress reactivity in pregnancy is limited. This paper reviews the current state of this literature with an emphasis on gaps in knowledge and future directions.

Psychoneuroimmunology in pregnancy: immune pathways linking stress with maternal health, adverse birth outcomes, and fetal development

It is well-established that psychological stress promotes immune dysregulation in nonpregnant humans and animals. Stress promotes inflammation, impairs antibody responses to vaccination, slows wound healing, and suppresses cell-mediated immune function. Importantly, the immune system changes substantially to support healthy pregnancy, with attenuation of inflammatory responses and impairment of cell-mediated immunity. This adaptation is postulated to protect the fetus from rejection by the maternal immune system. Thus, stress-induced immune dysregulation during pregnancy has unique implications for both maternal and fetal health, particularly preterm birth. However, very limited research has examined stress-immune relationships in pregnancy. The application of psychoneuroimmunology research models to the perinatal period holds great promise for elucidating biological pathways by which stress may affect adverse pregnancy outcomes, maternal health, and fetal development.

Preconceptional stress and racial disparities in preterm birth: an overview

OBJECTIVE

We reviewed the evidence for three theories of how preconceptional psychosocial stress could act as a contributing determinant of excess preterm birth risk among African American women: early life developmental plasticity and epigenetic programming of adult neuroendocrine systems; blunting, weathering, or dysfunction of neuroendocrine and immune function in response to chronic stress activation through the life course; individuals' adoption of risky behaviors such as smoking as a response to stressful stimuli.

METHODS

Basic science, clinical, and epidemiologic studies indexed in MEDLINE and Web of Science databases on preconceptional psychosocial stress, preterm birth and race were reviewed.

RESULTS

Mixed evidence leans towards modest associations between preconceptional chronic stress and preterm birth (for example common odds ratios of 1.2-1.4), particularly in African American women, but it is unclear whether this association is causal or explains a substantial portion of the Black-White racial disparity in preterm birth. The stress-preterm birth association may be mediated by hypothalamic-pituitary-adrenal axis dysfunction and susceptibility to bacterial vaginosis, although these mechanisms are incompletely understood. Evidence for the role of epigenetic or early life programming as a determinant of racial disparities in preterm birth risk is more circumstantial.

CONCLUSIONS

Preconceptional stress, directly or in interaction with host genetic susceptibility or infection, remains an important hypothesized risk factor for understanding and reducing racial disparities in preterm birth. Future studies that integrate adequately sized epidemiologic samples with measures of stress, infection, and gene expression, will advance our knowledge and allow development of targeted interventions.

Pre-pregnancy stress reactivity and pregnancy outcome

Stress has been proposed as a cause of preterm birth (PTB) and small-for-gestational age (SGA), but stress does not have the same effects on all women. It may be that a woman's reaction to stress relates to her pregnancy health, and previous studies indicate that higher reactivity is associated with reduced birthweight and gestational age. The objective of this study was to examine the relationship between pre-pregnancy cardiovascular reactivity to stress and pregnancy outcome. The sample included 917 women in the Coronary Artery Risk Development in Young Adults Study who had cardiovascular reactivity measured in 1987-88 and at least one subsequent singleton livebirth within an 18-year period. Cardiovascular reactivity was measured using a video game, star tracing and cold pressor test. Gestational age and birthweight were based on the women's self-report, with PTB defined as birth <37 weeks' gestation and SGA as weight <10th percentile for gestational age. Linear and Poisson regression and generalised estimating equations were used to model the relationship between reactivity to stress and birth outcomes with control for confounders. Few associations were seen between reactivity and pregnancy outcomes. Higher pre-pregnancy diastolic blood pressure (adjusted relative risk 1.14; 95% confidence interval [CI] 0.98, 1.34) and mean arterial pressure reactivity (1.15; 0.98, 1.36) were associated with risk of PTB at first pregnancy, while SGA was associated with lower systolic blood pressure reactivity (0.76; 0.60, 0.95). No associations were seen with other measures of reactivity. Contrary to hypothesis, the association between heart rate reactivity and PTB in first pregnancy was stronger in whites (adjusted relative risk 1.39; 1.03, 1.88) than in blacks (1.00; 0.83, 1.20; P for interaction = 0.08). Similar results were found for mean arterial pressure. No strong associations were found between higher pre-pregnancy stress reactivity and SGA or PTB, and stress reactivity did not have a stronger association with birth outcomes in blacks than whites.

What causes racial disparities in very preterm birth? A biosocial perspective

Very preterm birth (<32 weeks' gestation) occurs in approximately 2% of livebirths but is a leading cause of infant mortality and morbidity in the United States. African-American women have a 2-fold to 3-fold elevated risk compared with non-Hispanic white women for reasons that are incompletely understood. This paper reviews the evidence for the biologic and social patterning of very preterm birth, with attention to leading hypotheses regarding the etiology of the racial disparity. A systematic review of the literature in the MEDLINE, CINAHL, PsycInfo, and EMBASE indices was conducted. The literature to date suggests a complex, multifactorial causal framework for understanding racial disparities in very preterm birth, with maternal inflammatory, vascular, or neuroendocrine dysfunction as proximal pathways and maternal exposure to stress, racial differences in preconceptional health, and genetic, epigenetic, and gene-environment interactions as more distal mediators. Interpersonal and institutionalized racism are mechanisms that may drive racially patterned differences. Current literature is limited in that research on social determinants and biologic processes of prematurity has been generally disconnected. Improved etiologic understanding and the potential for effective intervention may come with better integration of these research approaches.

Evaluation of preterm births and birth defects in liveborn infants of US military women who received smallpox vaccine

BACKGROUND

Women serving in the US military have some unique occupational exposures, including exposure to vaccinations that are rarely required in civilian professions. When vaccinations are inadvertently given during pregnancy, such exposures raise special concerns. These analyses address health outcomes, particularly preterm births and birth defects, among infants who appear to have been exposed to maternal smallpox vaccination in pregnancy.

METHODS

This retrospective cohort study included 31,420 infants born to active-duty military women during 2003-2004. We used Department of Defense databases to define maternal vaccination and infant health outcomes. Multivariable regression models were developed to describe associations between maternal smallpox vaccination and preterm births and birth defects in liveborn infants.

RESULTS

There were 7,735 infants identified as born to women ever vaccinated against smallpox, and 672 infants born to women vaccinated in the first trimester of pregnancy. In multivariable modeling, maternal smallpox vaccination in pregnancy was not associated with preterm or extreme preterm delivery. Maternal smallpox vaccination in the first trimester of pregnancy was not significantly associated with overall birth defects (OR 1.40; 95% CI: 0.94, 2.07), or any of seven specific defects individually modeled.

CONCLUSIONS

Results may be reassuring that smallpox vaccine, when inadvertently administered to pregnant women, is not associated with preterm delivery or birth defects in liveborn infants.

Hostility and anomie: links to preterm delivery subtypes and ambulatory blood pressure at mid-pregnancy

Underlying maternal vascular disease has been implicated as one of several pathways contributing to preterm delivery (PTD) and psychosocial factors such as hostility, anomie, effortful coping, and mastery may be associated with PTD by affecting maternal vascular health. Using data from the Pregnancy Outcomes and Community Health (POUCH) study, we included 2018 non-Hispanic White and 743 African American women from 52 clinics in five Michigan, USA communities. Women were interviewed at 15-27 weeks' gestation and followed to delivery. We found that relations between psychosocial factors and PTD subtypes (i.e. medically indicated, premature rupture of membranes, spontaneous labor) varied by race/ethnicity and socio-economic position (Medicaid insurance status). Among African American women not insured by Medicaid, anomie levels in mid-pregnancy were positively associated with medically indicated PTD after adjusting for maternal age and education. Among all women not insured by Medicaid, hostility levels were positively associated with spontaneous PTD after adjusting for maternal race/ethnicity, age, and education. Failure to detect links between psychosocial factors and PTD risk in poorer women may be due to their excess risk in multiple PTD pathways and/or a more complex web of contributing risk factors. In a subset of 395 women monitored for blood pressure, anomie scores were positively associated with systolic blood pressure and heart rate and hostility scores were positively associated with systolic and diastolic blood pressure, heart rate and mean arterial pressure in models that included time, awake/asleep, race/ethnicity, and age as covariates. Further adjustment for body mass index and smoking attenuated the anomie-vascular relations but had little effect on the hostility-vascular relations. Overall this study of pregnant women provides some physiologic evidence to support findings linking levels of anomie and hostility with risk of PTD.

Epidemiology and environmental factors in preterm labour

An epidemiological and environmental approach is the appropriate starting point to understanding preterm labour. Although there are multiple aetiologies it seems likely that anthropometric and environmental risk factors in combination with inherent genetic susceptibilities contribute to an increased risk of preterm labour for certain women. Poct 2pulation-based studies identifying risk factors and quantifying outcomes facilitate informed counselling and provide a framework for developing prediction tools. Carefully conducted case-control and cohort studies identify associations that may contribute to an understanding of causation. A combined approach encompassing epidemiology, pathophysiology and clinical research is required to understand the aetiologies, prevention and optimal management of preterm labour. This review focuses on the epidemiology of preterm labour and the role of environmental factors.