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ELECTRONIC ARTICLE |
From the Department of Society, Human Development, and Health, Harvard School of Public Health, Boston, Massachusetts
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Methods. Logistic regression analyses of the Detail Natality Data, 1998 (n = 2 436 890), were used to examine differentials in LBW by nativity across racial/ethnic groups and by education level.
Results. Although foreign-born status does not protect against LBW among white women (95% confidence interval [CI]: 0.96, 1.03) and it increases the risk among Asian women by 24% (95% CI: 1.13, 1.36), it reduces the risk by 
25% among black women (95% CI: 0.72, 0.78) and by 19% among Hispanic women (95% CI: 0.78, 0.84). By educational attainment, for whites, blacks, and Hispanics the protective effect of foreign-born status is stronger among women with low education (ie, 0–11 years) than among women with more education. The educational gradient in LBW is less pronounced among foreign-born white, black, and Hispanic women than among their US-born counterparts.
Conclusions. Foreign-born status is associated with LBW. The direction and strength of this association varies across racial/ethnic groups, and within those groups it varies by educational level. Future research may test hypotheses regarding the mechanisms underlying these variations in LBW, including health selection of immigrants, cultural factors, social support, and social environment.
Key Words: low birth weight • immigrants • race/ethnicity • socioeconomic status
Abbreviations: LBW, low birth weight • SES, socioeconomic status • OR, odds ratio • CI, confidence interval
Preventing low birth weight (LBW) and diminishing disparities in LBW among racial/ethnic groups are significant public health goals.1–3 The importance of understanding racial/ethnic differences in LBW is highlighted by a 2003 National Institutes of Health request for research proposals in this area: "Reducing Preterm and Low Birthweight in Minority Families" (PA-04-027).3 The National Institutes of Health has called for research that can elucidate the mechanisms that underlie racial/ethnic disparities in LBW as well as interventions to reduce them. A first step in this direction is epidemiologic work with existing data to uncover the extent and sources of variation in LBW among and within various racial/ethnic groups.
This research was undertaken to provide insight into 3 critical issues in the area of LBW within the context of the United States: (1) whether foreign-born status confers a protective effect against LBW; (2) whether this protective effect varies across racial/ethnic groups (non-Hispanic whites, non-Hispanic blacks, non-Hispanic Asians, and Hispanics [of any race]); and (3) whether the effect of foreign-born status varies by socioeconomic status (SES), ie, education, within various racial/ethnic groups. In this article, we use the terms foreign-born and immigrant interchangeably.
Past studies of LBW among immigrants have documented that, given similar SES and clinical risk factors, in some areas of the United States (eg, Arizona, California, Colorado, Florida, Illinois, New Jersey, New Mexico, New York, Texas) foreign-born women are less likely to have LBW infants than their US-born counterparts.4–19 However, this result varies across racial/ethnic and national-origin groups.4–19 Using 1985–1987 national data, a previous study documented that there were significant differentials between US-born and foreign-born women in LBW and that these differentials varied across major US racial/ethnic groups.20 Our article contributes to this body of research by using more recent (ie, 1998) national data to address the effects of nativity, race/ethnicity, and SES, and their interaction, on LBW.
A central issue in immigrant health studies is the choice of an appropriate reference group. Some studies have compared immigrants with the majority (ie, US-born, non-Hispanic white) population,8 whereas others have compared immigrants to their racial/ethnic group US-born counterparts (eg, foreign-born Mexicans to US-born Mexicans).16 Recent sociological research on immigrant adaptation suggests that both comparisons are important. Although assimilation to the white majority remains a possible pathway, preservation of ethnic identity and assimilation into a US-born ethnic minority group constitutes an alternative pathway.21–24 Therefore, this study sequentially presents both comparisons.
In addition to considering the effect of foreign-born status, we systematically examine the effect of SES on LBW among various racial/ethnic groups as well as its interaction with foreign-born status. Our objective is twofold. First, we examine whether the effect of foreign-born status varies by education level. Second, we assess whether the education gradient in LBW varies by race/ethnicity and nativity.
The interaction effect of SES and nativity on LBW is closely related to the "epidemiologic paradox," a salient term in immigrant health that denotes that, despite having lower SES, foreign-born women tend to have better birth outcomes than US-born women.5,7,9,16,25–29 Epidemiologic paradoxes such as this one are sometimes defined in relation to the average SES of a population. In other cases, the term "paradox" is used to denote a residual protective effect of foreign-born status that cannot be accounted for by measured demographic, socioeconomic, behavioral, and/or medical risk factors. We believe that a better test of a paradoxical effect is whether foreign-born status confers a protective effect (particularly) among individuals of low SES. Therefore, we test for an interaction effect between foreign-born status and education.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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5 lb, 8 oz. The sample was further restricted to women 
20 years old because of the complexity of factors influencing prenatal outcomes in younger mothers.30 This study used data from the 1998 Detail Natality data set,31 which includes 3 945 192 births within the United States to US-born and foreign-born women from all states, territories, and the District of Columbia. After excluding births to mothers <20 years old (494 622), nonsingleton births (181 374), those records for which maternal nativity was missing (10 156), and those births to individuals not residing in the United States (3639), our sample was 3 307 684. Excluding all observations for which the covariates of interest were missing reduced the sample further to 2 436 890, largely because of the exclusion of observations for which tobacco (ie, smoking [20.5%]) and alcohol (15%) use during pregnancy was missing. With the exception of prenatal care (4.9% missing), the percent missing for all the other covariates did not exceed 1.4%. Smoking and drinking alcohol showed large differential missing rates between US-born and foreign-born women (10.8% vs 31.1% and 16.9% vs 34.2%, respectively). Also, missing rates for these variables were higher among Asian and Hispanic mothers than among white and black mothers. Both patterns resulted primarily from large missing rates for California and New York, 2 states with large concentrations of Asian and Hispanic immigrants.
Among both US-born and foreign-born women, the LBW rate was lower for those with missing information on smoking than for those with complete information (4.9% vs 5.7% and 4.5% vs 5.2%, respectively; P < .001). Similarly, among both US-born and foreign-born women, the LBW rate was lower for those with missing information on drinking than for those with this information (4.8% vs 5.7% and 4.4% vs 5.2%, respectively; P < .001). However, the differential between US-born and foreign-born women for whom smoking and drinking statuses were missing was comparable to the respective differential among women for whom this information was recorded. The LBW rate ratio of foreign-born women to US-born women was 0.92 for those with missing information on smoking and 0.91 for those with information on smoking. Similarly, for drinking, the respective LBW rate ratios were 0.92 (missing) and 0.91 (not missing). In sum, although the LBW rates for both foreign-born and US-born women are higher in the portion of the data used in this analysis (ie, after excluding women with missing information on smoking and drinking) than in the original data, the magnitude of the LBW rate differential between these 2 groups is similar. Therefore, our results on the estimation of the effect of foreign-born status on LBW are not likely to be biased. As described below, we confirmed this by conducting all the analyses reported here on the complete data without controlling for tobacco and alcohol use (results not shown).
Variables
The outcome variable was LBW and coded as "1" if the infant was <2500 g and coded as "0" if the infant was 2500 g. Demographic variables included maternal age, race/ethnicity, nativity (ie, place of birth), and marital status. Maternal age was characterized as 20 to 34 and 35 years to allow for the assessment of increased risk of LBW in the older age group. The mothers' self reported race, ethnicity, and birthplace were used to characterize US-born and foreign-born non-Hispanic whites, non-Hispanic blacks, non-Hispanic Asians, and Hispanics (of all races). Marital status of the mothers compared "married" to "other." The mother's SES was characterized by education (0–11, 12, 13–15, and 16 years).
Additionally, the mothers' information included prenatal care, health behaviors, and complications during pregnancy. Prenatal care was measured by using the Kessner index, which is based on the month care began, the number of visits made, and the length of gestation. This measure accounts for women with short gestations who have less time to receive prenatal care than do women with long gestations. Therefore, the Kessner index is a better assessment of prenatal care use than either the timing of the first visit or the number of visits alone. This variable was dichotomized as receiving adequate care versus intermediate and inadequate care.
Health behaviors during pregnancy included tobacco and alcohol use. Controlling for these behaviors introduced some complexity into the study, because California, Indiana, South Dakota, and New York (partial data collection: 50%) did not collect information on tobacco use in 1998. Additionally, California and South Dakota did not collect information on alcohol use. Given the high proportion of immigrants in California and New York, excluding women for whom tobacco and alcohol use were not recorded could potentially bias our results. Excluding observations with missing data on tobacco and alcohol for California resulted in losing 30.2% of the foreign-born women in the original sample. Excluding the respective observations for New York represented losing only 2.7% of the foreign-born women in the original data because of a higher rate of reporting tobacco and alcohol use among foreign-born women in New York than among US-born women. On the other hand, not controlling for tobacco and alcohol use would prevent us from assessing whether health behaviors during pregnancy account for some of the protective effect of foreign-born status on LBW. To address these issue, our first analysis was restricted only to women with complete information on tobacco and alcohol use, whereas our second analysis comprised the total population but models did not control for behavioral risk factors. Here we present the results of the first analysis, because the effect of foreign-born status was similar in both analyses and it was useful to control for health behaviors. Additionally, regarding racial/ethnic variations in the effect of foreign-born status, the results presented in this article, which exclude California, are comparable to those of Fuentes-Afflick et al,4–6,8,9 who have published a series of studies of LBW using exclusively data for that state.
Chronic hypertension, anemia, diabetes, and pregnancy-associated risk factors such as preeclampsia, eclampsia, placenta previa, and placenta abruption were used to measure medical risk factors. The prevalence of these risk factors is different across racial/ethnic and nativity groups (Table 1), which suggests the importance of controlling for them in statistical analyses.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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30% lower than among the US-born. Among Hispanics, the LBW rate is 20% lower.
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Risk factors for LBW vary by race/ethnicity and nativity (Table 1). For example, in all racial/ethnic groups, foreign-born women are less likely to be single mothers than their US-born counterparts. This difference is particularly strong among black women (ie, although only 35% of US-born black women are married, 63% of foreign-born black women are married). Regarding education, Hispanic women (both US-born and foreign-born) are more likely to have low education than women in any other racial/ethnic group. Foreign-born Asian and Hispanic women are considerably more likely to have low education (0–11 years) than their US-born counterparts (11.3% vs 4.8% among Asian women and 55.9% vs 23.7% among Hispanic women). Among all racial/ethnic groups, foreign-born women have much lower rates of smoking during pregnancy than their US-born counterparts. Use of adequate prenatal care does not exhibit strong differences between foreign-born and US-born women in any racial/ethnic group. With the exception of diabetes and placenta previa/abruption, medical risk factors tended to be lower among foreign-born than US-born women.
The Effect of Foreign-Born Status
Table 2 shows multivariate logistic analyses using the entire sample (n = 2 436 890). In model 1, after controlling for demographics, SES, prenatal care, health behaviors during pregnancy, and clinical risk factors, women of racial/ethnic minority groups had significantly higher odds of having an LBW infant than non-Hispanic white women (2.36 for black women, 1.59 for Asian women, and 1.17 for Hispanic women). After controlling for foreign-born nativity in model 2, the odds ratios (ORs) with respect to white women increased for all racial/ethnic minority groups, especially for Asian and Hispanic women, ie, favorable birth outcomes among foreign-born women contribute to lowering the gap between racial/ethnic minorities and white women. In model 2, the overall OR for foreign-born women vis-à-vis US-born women is 0.85 (95% confidence interval [CI]: 0.83, 0.87), suggesting that, on average, foreign-born women are 15% less likely to have a LBW infant.
The Effect of Foreign-Born Status by Race/Ethnicity
In model 3, we introduced an interaction between race/ethnicity and nativity to examine whether the protective effect of foreign-born status varies by racial/ethnic group. The interaction is significant, and as illustrated by the ORs, the protective effect of foreign nativity represents a reduction in the risk of LBW of 23% among black (95% CI: 0.74, 0.80) and Hispanic (95% CI: 0.75, 0.80) women, whereas foreign nativity does not have a protective effect among white women (95% CI: 0.95, 1.03) and has an adverse effect among Asian women (95% CI: 1.18, 1.41).
Because model 3 (Table 2) showed a significant differential effect of foreign-born nativity across racial/ethnic groups in Table 3, we present statistical models stratified by race/ethnicity controlling for the same factors as in Table 2. Model 4 shows the effect of demographic and socioeconomic factors, prenatal care, health behaviors during pregnancy, and clinical risk factors for each racial/ethnic group, without including foreign-born status. As shown in Fig 3, although LBW exhibits a strong, monotonic education gradient among (all) white women, such a gradient is less pronounced for black women and weak and nonmonotonic for Asian and Hispanic women.
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25% among black women (95% CI: 0.72, 0.78) and by 19% among Hispanic women (95% CI: 0.78, 0.84).
The Effect of Foreign-Born Status by Race/Ethnicity and Education
In model 6 (Table 4) we introduce an interaction between foreign-born status and education to examine whether the effect of foreign-born nativity varies by race/ethnicity and SES. The interaction is significant for all groups. As shown in Fig 4, for white, black, and Hispanic women, the effect of foreign-born status is especially protective among women with low (ie, less than high school) education (0–11 years). Although foreign-born white women with 0 to 11 years of education are 18% less likely to have an LBW infant than their US-born counterparts (95% CI: 0.74, 0.92), foreign-born white women with 12 (95% CI: 0.91, 1.03), 13–15 (95% CI: 0.90, 0.1.05), and 16 (95% CI: 1.02, 1.16) years of education do not have an advantage over their US-born counterparts. Among black and Hispanic women, there is an inverse socioeconomic gradient in the protective effect of foreign-born nativity, ie, foreign-born status is associated with a stronger protection against LBW among women with low education, and this protective effect decreases with increasing education (Fig 4). Among black women, the overall pattern is one of diminishing returns of foreign-born status as education increases. Among Hispanic women, foreign-born status is protective against LBW among women in the 2 lowest education groups. Two main differences between black and Hispanic women emerge. First, at every education level, the protective effect of foreign-born status is stronger for black women than for Hispanic women. Second, although for Hispanic women foreign-born status is not protective among those in the 2 highest educational levels, for black women foreign-born status significantly reduces the odds of LBW by 20% among women those in those educational groups. Among Asian women, the adverse effect of foreign-born status does not follow a clear pattern across various educational groups.
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16 years) for each racial/ethnic group broken down by nativity. There is a pronounced educational gradient in LBW for US-born white, black, and Hispanic women and for foreign-born white women. The educational gradient is flat for foreign-born Hispanic women and indistinct for foreign-born black women and US-born and foreign-born Asian women.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Foreign-born white, black, and Hispanic women are less likely to have LBW infants than their US-born counterparts. Immigrant women exhibit lower rates of known risk factors for LBW (eg, low education, single motherhood, smoking and drinking during pregnancy) than their US-born counterparts. However, after controlling for demographics, SES, prenatal care, health behaviors, and medical risk factors, our models showed a significant protective effect of foreign-born status. With the present data, we cannot evaluate whether unmeasured risk factors for LBW, cultural factors, or the selection of healthy immigrant women influence this pattern. Prior research has suggested that protective cultural factors and social support among (Latino) immigrants may explain the protective effect of foreign nativity.12,27,32,33 Another plausible explanation is that foreign-born women may be selected for being healthy.25,26,29,34 Additional research is needed to examine these possible mechanisms.
Additionally, our results showed a significant independent effect of education. Smoking, drinking, and prenatal care could presumably mediate the effect of education, because more educated women may be more aware of the harmful effects of these behaviors and positive effects of adequate prenatal care. Therefore, unmeasured factors may account for the observed (residual) effect of education. Other research showed that smoking and drinking mediated the effect of education, whereas income had a significant independent effect.35 It has been suggested that in studies of racial/ethnic disparities in maternal and infant health, the SES measures should be outcome and population specific and chosen on conceptual grounds.36,37 However, as in the present research, this is often restricted by the limited availability of SES information. We were unable to determine if the effect of education is confounded with the effect of other facets of SES such as income, occupation, and/or health insurance, because education was the only available SES indicator.
For white, black, and Hispanic women, foreign-born status has a protective effect against LBW, particularly among women with low education (Fig 4). Given that 63% of Hispanic women in the sample are foreign-born and that, among those, 56% have less than a high school education (a much larger proportion than in any other racial/ethnic/nativity group), the relatively low rates of LBW in this group are especially significant.
Educational gradients in LBW are less pronounced among foreign-born white, black, and Hispanic women than among their US-born counterparts (Figs 2 and 5). LBW does not exhibit education gradients among Asian women. Some previous studies found that the education gradient varied by nativity/acculturation level. For example, an analysis of the Hispanic Health and Nutrition Examination Survey12 found that the effect of education on LBW depended on the level of acculturation. Education was unrelated to LBW risk among women with a "Mexican cultural orientation," whereas increased education was associated with reduced LBW risk among those with a "US cultural orientation." Gould et al7 found that although maternal education was protective against LBW among US-born, non-Hispanic white and black women in California, it was not protective among foreign-born Mexican and Asian Indian women.
The issue of differential SES (eg, educational) gradients by race/ethnicity and nativity remains an important question for future research, ie, why does low education (compared with higher education) increase the risk of LBW more among US-born women than among foreign-born women? Are there cultural factors that protect foreign-born women with low education, and/or does health selection among immigrants play a role?
Possible Mechanisms
This article contributes to the literature on racial/ethnic and socioeconomic disparities in health by showing that the effect of nativity on LBW varies significantly by race/ethnicity and SES. Additional research is needed to examine why foreign-born status confers a protective effect and why education gradients in LBW are less pronounced among the foreign-born than among the US-born. For instance, on average, immigrants may have better health than those in their country of origin who do not migrate and those immigrants who return to their country of origin. Using health data for Mexico and the United States, Soldo et al34 showed that Mexican immigrants in the United States have a better health profile than their counterparts who returned to Mexico. Ideally, to explore the issue of selection, we would like to compare rates of LBW among foreign-born women from a given country of origin with their US-born ethnic counterparts, as well as with comparable women in their country of origin, including those who have never migrated and return migrants.
If immigrant women are selected for being healthier or having better health behaviors across education levels, such health selection may override the education gradient. If, as suggested by Jasso et al,29 there is a minimum health level that would make migration worthwhile, this may limit the dispersion in health outcomes among immigrants, thus flattening SES gradients. An alternative hypothesis, which has been proposed primarily in regard to immigrants of Latino descent, is that immigrant women may exhibit cultural factors protective against LBW.12,27,32,33,38,39 This conjecture is often presented in association with an acculturation hypothesis, ie, that there is an erosion of such protective factors with time spent in the US (within 1 generation) and across generations, which results in a deterioration of health outcomes. In health research, the acculturation hypothesis is frequently tested by examining whether higher levels of "acculturation" (operationalized as nativity status [ie, US-born versus foreign-born], length of stay in the United States [if foreign-born], generation in the United States, and English language ability) are associated with unfavorable health outcomes.40–42 If present across educational levels, protective cultural factors may attenuate the education gradient. Another possible explanation is that the education gradient in the immigrants' countries of origin may be different (eg, less pronounced) than in the United States. To explore the last hypothesis, it would be necessary to have data that allow a breakdown by national origin as well as data on LBW by education in the immigrants' countries of origin. Increasingly, health researchers realize that a meaningful examination of immigrant health will require health data on the origin and destination countries.29,34,43
Limitations
The present research has several limitations arising from omissions in the available data. First, as discussed above, the present data have limited power to examine possible mechanisms that may underlie variations in LBW by race/ethnicity, nativity, and SES. However, uncovering such variations may suggest fruitful hypotheses for future research. For instance, studies may explore why black women, irrespective of education level and nativity status, are more likely to have LBW infants. Comparisons between US-born and foreign-born black women may help address the role of psychosocial stress associated with exposure to discrimination.3 Similarly, such comparisons may provide additional tests for the "weathering hypothesis."44–46 For instance, does older maternal age increase the risk of LBW more among US-born black women than among foreign-born black women, given that the former may have longer exposure to factors contributing to weathering such as poverty and discrimination?
Also, why do foreign-born Hispanic women with limited education have low rates of LBW? For example, is the social (eg, neighborhood) environment experienced by this group less disadvantaged than that experienced by their counterparts from other racial/ethnic backgrounds? Recent research has suggested that among Latino women in Los Angeles, California, both individual-level foreign-born status and immigrant enclaves at the neighborhood level have protective effects on LBW.47 Alternatively, there may be buffers (eg, social support) that protect this group against stressors in their social environment.
Regarding Asian women, what factors may account for the relatively high LBW rate among immigrants vis-à-vis the US-born? For example, this pattern may be driven by certain national-origin subgroups, among which refugees constitute a large proportion of the foreign-born, because unlike voluntary immigrants, refugees may not be selected for positive health and indeed may disproportionately suffer from physical and mental health problems.
Second, although we were able to distinguish foreign-born from US-born mothers within various racial/ethnic groups, we were not able to distinguish second-generation immigrants (ie, those born in the United States of foreign-born parents). We are unable to explore whether the favorable birth-weight outcomes among first-generation immigrants (ie, the protective effect of foreign-born status against LBW) persist among the second generation. From a demographic standpoint, the second generation is becoming increasingly prominent. Births in the United States are surpassing immigration as the main source of growth among immigrants from Latin America, and as a consequence, the second generation will surpass the first generation in size by 2020.48 Current sociological research also suggests the need for examining the outcomes of the second generation when assessing the adaptation trajectories of various immigrant groups21–24,28,49–51.
Third, given the national diversity among immigrants, it would be advisable to break down the foreign-born by country of origin. Prior state-level studies have shown that the protective effect of foreign-born status varies across national-origin groups.5,6 However, we believe that a first, comprehensive, national-level analysis should examine LBW for the 4 major racial/ethnic groups. Then, finer analyses by national origin may examine the extent to which specific subgroups deviate from the average pattern from their racial/ethnic group
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CONCLUSIONS |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address correspondence to Dolores Acevedo-Garcia, PhD, MPA-URP, Department of Society, Human Development, and Health, Harvard School of Public Health, 677 Huntington Ave, Kresge 722, Boston, MA 02115-6096. E-mail: dacevedo{at}hsph.harvard.edu
No conflict of interest declared.
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REFERENCES |
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This article has been cited by other articles:
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  T. L. Osypuk and D. Acevedo-Garcia Are Racial Disparities in Preterm Birth Larger in Hypersegregated Areas? Am. J. Epidemiol., June 1, 2008; 167(11): 1295 - 1304. [Abstract] [Full Text] [PDF]
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 N Auger, Z-C Luo, R W Platt, and M Daniel Do mother's education and foreign born status interact to influence birth outcomes? Clarifying the epidemiological paradox and the healthy migrant effect J. Epidemiol. Community Health, May 1, 2008; 62(5): 402 - 409. [Abstract] [Full Text] [PDF]
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 R. T. Kimbro, S. Bzostek, N. Goldman, and G. Rodriguez Race, Ethnicity, And The Education Gradient In Health Health Aff., March 1, 2008; 27(2): 361 - 372. [Abstract] [Full Text] [PDF]
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 M. R. Decker, A. Raj, and J. G. Silverman Sexual Violence Against Adolescent Girls: Influences of Immigration and Acculturation Violence Against Women, May 1, 2007; 13(5): 498 - 513. [Abstract] [PDF]
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 E.-J. W.M. Troe, A. E. Kunst, V. Bos, I. M. Deerenberg, I. M.A. Joung, and J. P. Mackenbach The effect of age at immigration and generational status of the mother on infant mortality in ethnic minority populations in The Netherlands Eur J Public Health, April 1, 2007; 17(2): 134 - 138. [Abstract] [Full Text] [PDF]
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