Multi-level modeling of social factors and preterm delivery in Santiago de Chile

Birth before the 37th week of gestation (preterm birth) is an important cause of infant and neonatal mortality [1]. Nonetheless, the etiology of spontaneous preterm birth is still largely unknown [2]. Populations demonstrate widely varying risks of the outcome, but the specific genetic, dietary, behavioral or psychosocial factors that contribute to this variation remain obscure [3]. Researchers have described consistent socioeconomic patterns in preterm delivery risk in developed countries [4]. These factors include maternal variables such as education [5] and income [6], as well as level of neighborhood factors such as deprivation [7] and crime [8]. Despite a substantial literature on social factors and preterm birth in the US and Europe, however, there is little research reported from middle income countries. Literature searches revealed a few articles from the developing world on social factors in relation to infant mortality [9], low birthweight or other adverse reproductive events [10], but the relation between preterm delivery and social variables appears to be largely unstudied outside of wealthy societies.

Chile is a Latin American nation with just over 16 million inhabitants, about 85% of which are urbanized, with about 40% of the population living in the capital district of Santiago. It is representative of a large number of nations outside of North America and Europe that are rapidly urbanizing, integrating women into the formal economy and experiencing an epidemiologic and demographic transition away from high levels of childhood mortality and toward an older population age structure and higher burden of chronic diseases. The World Bank currently includes 96 nations as "middle income" nations – gross national income (GNI) per capita of more than $875 but less than $10,726 – including Chile, which had a 2005 GNI of about $6000 [11]. Although these 96 nations comprise about half of the world's population, epidemiology of preterm birth in relation to social factors has been studied very little to date in this social and economic context.

We therefore set out to estimate the relation between maternal social status and neighborhood social status on risk of preterm delivery in an urbanized region of a middle income country, as well as the joint relations between these factors in multilevel regression analyses. We used maternal education as an indicator of individual social status and an index created from decennial census data as an indicator of neighborhood social status. By geocoding maternal addresses from birth certificates, we were able to assign exposure values to births as recorded in vital records data.

Our study of individual education and community SES in relation to preterm birth in a middle-income country demonstrated detectable effects that were nevertheless much more modest in magnitude than what is commonly observed in the US and Europe. Figure 1 compares data from the current study with pooled data from eight urban centers in the US on a standardized axis for the exposure [7]. Data from Chile have been restricted to n = 49,771 women aged 20 years and older for great comparability with the US data. It is important to note that the absolute level of economic development at a specific value for this index is not comparable between Chile and the US, but rather this is a comparison at equal points in the distribution of scores in each country. If one views the African-American population as an "under-developed" population within the US (in the sense that as a group there has been less investment in human capital and neighborhood infrastructure), then this graph suggests a continuum of economic levels in which neighborhood SES would become increasingly important as the average level of development of the population increases. This represents an intriguing hypothesis that could be investigated in subsequent studies in a greater variety of economic contexts. Furthermore, this comparison shows that although the Non-Hispanic White US women studied have a greater slope of neighborhood deprivation, their average risk of preterm delivery is roughly comparable to that observed in Santiago. These US women therefore have considerably greater heterogeneity in risk by neighborhood than the women from Chile.

These results are also consistent with the pattern of adverse outcomes observed in previous studies for Hispanic migrants in the US [21, 22]. Risk of adverse birth outcomes is consistently seen to be modest in Hispanic US women, a phenomenon often referred to as the "Hispanic Paradox" [23]. Moreover, Hispanic women in the US have much more modest SES gradients of risk than do women who identify as Non-Hispanic White and Non-Hispanic Black. This is especially true for foreign-born Hispanic women [22]. The current analysis shows that for preterm delivery, the pattern among women in Santiago, Chile is indistinguishable from that seen for these foreign-born US Hispanic women in this regard, which is perhaps not unsurprising. Most previous literature on this topic considers gradients in risk associated with educational attainment of the mother, since this is a variable widely available in US vital statistics data. The present study shows that this applies equally well to neighborhood deprivation, as measured by census data. Although few published studies have considered risk of preterm delivery for US Hispanic women by neighborhood socioeconomic characteristics, Masi et al. similarly observed no important influence of neighborhood economic characteristics on preterm delivery for Hispanic women in Chicago [24].

The surprisingly healthy profile of birth risk in Chile compared to the US, despite being a much poorer country, may reflect substantial differences in the organization of medical services for pregnant woman. The current program for universal provision of prenatal care was developed by the Ministry of Health in the late 1970s. There are no official statistics for the actual coverage of this ostensibly obligatory prenatal care, due to the fact that many women who qualify for public health insurance (FONASA) choose to instead attend private health care providers during pregnancy. In 1983 an early estimate of national coverage showed that 91% of pregnant women had prenatal care throughout pregnancy [25]. A later study conducted in the largest public hospital in Santiago in 1991–1992 showed that only 1.2% of the deliveries failed to receive required prenatal care control [26]. More contemporary analyses based on 2005 data from FONASA and the Health Statistics and Information Department of the Ministry of Health shows that the prenatal care program has a coverage of at least 98% of the population that is affiliated to FONASA in the Metropolitan Region around Santiago. Moreover, nearly 78% of those women are estimated to initiate prenatal control by a clinical provider before the end of the first trimester [27, 28].

The Chilean prenatal care program prides itself on an integrated multidisciplinary perspective with the intention of providing for complete physical and mental health of the mother during pregnancy and the mother-child pair after parturition. The program includes at least 6 professional visits by clinical nurse midwifes and/or physicians, referral to tertiary health care facilities if needed, educational sessions, laboratory exams including blood and urine monitoring (e.g., glucose blood level) as well as glucose tolerance testing, and screening for HIV, syphilis, Chagas disease, and cervical cancer. Women also receive three ultrasound examinations around gestational weeks 14, 24 and 34. Dietary supplements are also included and distributed according the nutritional status of the mother [29]. All of these services are intended to be universal, available locally via a practitioner who has an ongoing relationship to the pregnant woman, and completely covered by universal public health insurance (FONASA). While severe social inequalities exist in many other sectors of the Chilean health system [30], services for pregnant women are intended to be completely free of access barriers due to cost, travel distance or bureaucratic hurdles. While this goal may not always be achieved, it seems that the structure of the system is inherently more egalitarian in intent and more broad in scope than many of its North American counterparts. Recent randomized intervention studies in the US support the hypothesis that integrated prenatal care services can have a substantial beneficial effect on pregnancy outcomes [31].

The observed modest socioeconomic gradient in preterm birth in these data does not imply socioeconomic equality in health more generally. Chile remains a profoundly unequal society in many respects, with one of the highest gini coefficients in the world for income inequality (56.5, compared to 40.8 in the US) [[32], p. 332]. Other health outcomes appear to reflect these inequalities more directly, again pointing to the somewhat unique attention to healthy pregnancy as a long-standing Chilean policy priority. For example, deaths of infants in the first year of life in Chile fell from 17.0 per 1000 live births in 1990 to 9.4 per 1000 live births in 2000 [33]. Despite the dramatic overall improvement, however, inequalities grew more severe in the social distribution of the remaining deaths. In 2003 data, for example, infant mortality was 29.7 per 1000 live births among uneducated mothers compared with only 5.3 per 1000 among those with 13 or more years of education [34]. The overall improvements in infant mortality in the decade following democratization were not accompanied by improvements in the already impressively low risks of preterm birth or low birthweight [33], however, suggesting that these mortality gains for newborns were produced by the introduction of improved medical services for infants rather than by improvements in more fundamental social causes of adverse outcomes such as poverty and malnutrition [35].

While these data appear consistent with a more modest effect of social factors on preterm birth at the individual and community level, there are potential alternate explanations. One might question data quality, but this seems the least likely explanation for the findings, given the consistently observed advantage for Chilean pregnancies and the resources and attention devoted to data integrity in Chile. A more plausible concern is that our a priori decisions about relevant census variables may have failed to capture true social variation between communities [36]. For example, census districts are not salient community divisions for women living in Santiago, and their boundaries are often arbitrary with respect to socially recognized neighborhoods [37]. We also fail to account for many other plausibly important psychosocially mediated exposures, including family and social support during pregnancy, diet, exercise and occupational stress [38]. Finally, the relatively low proportion of successfully geocoded births could conceivably obscure a more marked socioeconomic pattern in the complete population than is observed in the analytic data set. More comprehensive exposure assessment in middle-income nations will fill important gaps in our nascent understanding of reproductive health [39].

We conducted a study of maternal social status and neighborhood social status on risk of preterm delivery in a middle-income country, using vital statistics and geocoded decennial census data. Analysis of 56,970 births that occurred in 2004 in the metropolitan region of Santiago revealed a relatively advantaged pattern of preterm risk overall, with only 6.4% of births delivering before 37 weeks. Moreover, although associations were observed between risk of outcomes and individual and neighborhood factors, the magnitudes of these associations were much more modest than those reported in North America. This relatively flat socioeconomic gradient for adverse birth outcomes is similar, however, to that reported for Hispanic women in the United States, especially those that are born in other countries. We considered several potential explanations for the modest social gradient in risk observed in these data, including the potential impact of Chile's relatively egalitarian approach to universal prenatal care.