Birth Weight Paradox Research Articles

The immigrant birthweight paradox in an urban cohort: Role of immigrant enclaves and ambient air pollution.

Background:Foreign-born Black and Latina women on average have higher birthweight infants than their US-born counterparts, despite generally worse socioeconomic indicators and prenatal care access, i.e. ‘immigrant birthweight paradox’ (IBP). Residence in immigrant enclaves and associated social-cultural and economic benefits may be drivers of IBP. Yet, enclaves have been found to have higher air pollution, a risk factor for lower birthweight.Objective:We investigated the association of immigrant enclaves and children’s birthweight accounting for prenatal ambient air pollution exposure.Methods:In the Boston-based Children’s HealthWatch cohort of mother-child dyads, we obtained birthweight-for-gestational-age z-scores (BWGAZ) for US-born births, 2006–2015. We developed an immigrant enclave score based on census-tract percentages of foreign-born, non-citizen, and linguistically-isolated households statewide. We estimated trimester-specific PM2.5 concentrations and proximity to major roads based residential address at birth. We fit multivariable linear regressions of BWGAZ and examined effect modification by maternal nativity. Analyses were restricted to non-smoking women and term births.Results:Foreign-born women had children with 0.176 (95% CI: 0.092, 0.261) higher BWGAZ than US-born women, demonstrating the IBP in our cohort. Immigrant enclave score was not associated with BWGAZ, even after adjusting for air pollution exposures. However, this association was significantly modified by maternal nativity (pinteraction = 0.014), in which immigrant enclave score was positively associated with BWGAZ for only foreign-born women (0.090, 95% CI: 0.007, 0.172). Proximity to major roads was negatively associated with BWGAZ (−0.018 per 10m, 95% CI: −0.032, −0.003) and positively correlated with immigrant enclave scores. Trimester-specific PM2.5 concentrations were not associated with BWGAZ.Significance:Residence in immigrant enclaves was associated with higher birthweight children for foreign-born women, supporting the role of immigrant enclaves in the IBP. Future research of the IBP should account for immigrant enclaves and assess their spatial correlation with potential environmental risk factors and protective resources.

The Role of Immigrant Enclaves and Ambient Air Pollution Exposure in the Immigrant Birthweight Paradox

BACKGROUND AND AIM: Foreign-born Black and Latina women have higher birthweight infants that their US-born counterparts, despite generally worse socioeconomic status and prenatal care access, i.e. immigrant birthweight paradox (IBP). Residence in immigrant enclaves and associated socio-cultural and economic benefits may be drivers of IBP. Yet, enclaves are known to have higher air pollution, a risk factor for lower birthweight. We investigated IBP accounting for both residential immigrant composition and air pollution exposure. METHODS: In the Boston-based Children’s HealthWatch cohort of non-smoking women-child dyads (n=2,116), we obtained birthweight-for-gestational age z-scores (BWGAZ) for US-born births from 2006-2015. Using maternal address, we estimated trimester-specific PM2.5 concentrations, proximity to major roadways, and immigrant orientation level based on census-tract percentages of foreign-born, non-citizen, and linguistically-isolated households (proxy for immigrant enclave). We fit multivariable linear regressions of BWGAZ by maternal nativity and sequentially adjusted for immigrant orientation and ambient air pollution sources. We also examined effect modification of IBP by immigrant orientation. RESULTS:Fifty-one percent of women were foreign-born. Immigrant orientation was negatively correlated with PM2.5 concentrations and proximity to major roads. Overall, children of foreign-born women had 0.19 (95% CI: 0.10, 0.28) higher mean BWGAZ than those of US-born mothers, demonstrating IBP in our cohort. BWGAZ did not significantly change after adjustment for immigrant orientation and air pollution sources. However, we found an additive effect of immigrant orientation on maternal nativity (p-interaction: ≤0.01), with IBP observed only in moderate (0.27, 95% CI: 0.09, 0.46) and high (0.29, 95% CI: 0.15, 0.44) immigrant-oriented tracts. CONCLUSIONS:Foreign-born women in higher immigrant-oriented areas had higher BWGAZ, supporting the role of immigrant enclaves on IBP. Prenatal air pollution exposure did not impact associations observed in this study. Future research should consider the role of immigrant enclaves and their spatial correlation with potential protective resources and environmental risk factors. KEYWORDS: Air pollution, Particulate matter, Birth outcomes, Environmental disparities, Exposures, Environmental epidemiology

The Obesity Paradox in Cardiovascular Diseases

The Obesity Paradox in Cardiovascular Diseases

Revisiting the low birthweight paradox using sibling data with implications for the classification of low birthweight.

We examined the birthweight threshold for increased odds of neonatal death among second births based on their elder sibling's birthweight category. This population-based cohort study included 190 575 women who delivered their first two non-anomalous singleton live births in Missouri (1989-2005). We examined the birthweight distribution and neonatal mortality curves of second births whose elder sibling had low versus adequate/high birthweight. We determined the optimal cut-off point for the classification of low birthweight among infants in each group based on the Youden index. Infants whose elder sibling had low birthweight had a lower mean birthweight and a higher percentage of low birthweight infants versus those whose elder sibling had adequate/high birthweight, but low birthweight infants in the former group had a lower rate of neonatal mortality. Upon standardizing the birthweight distribution to a Z-scale, neonatal mortality rates became comparable between the two groups at every rescaled birthweight for Z-scores ≥-3.7. The optimal cut-off point for low birthweight was 2500 and 3000 g among infants whose elder sibling had low and adequate/high birthweight, respectively. Using sibling data for the classification of LBW may enable the identification of average-sized infants who may be at increased risk of neonatal mortality.

Introduction: Gestation of the Birth Weight Paradox.

Introduction: Gestation of the Birth Weight Paradox.

Revisiting the ‘Low BirthWeight paradox’ using a model-based definition

IntroductionImmigrant mothers in Spain have a lower risk of delivering Low BirthWeight (LBW) babies in comparison to Spaniards (LBW paradox). This study aimed at revisiting this finding by applying a model-based threshold as an alternative to the conventional definition of LBW. MethodsVital information data from Madrid was used (2005–2006). LBW was defined in two ways (less than 2500g and Wilcox's proposal). Logistic and linear regression models were run. ResultsAccording to common definition of LBW (less than 2500g) there is evidence to support the LBW paradox in Spain. Nevertheless, when an alternative model-based definition of LBW is used, the paradox is only clearly present in mothers from the rest of Southern America, suggesting a possible methodological bias effect. ConclusionIn the future, any examination of the existence of the LBW paradox should incorporate model-based definitions of LBW in order to avoid methodological bias.

Patient citizens, immigrant mothers: Mexican women, public prenatal care and birth-weight paradox by Alyshia Gálvez

Patient citizens, immigrant mothers: Mexican women, public prenatal care and birth-weight paradox by Alyshia Gálvez

AlyshiaGalvez. Patient Citizens, Immigrant Mothers: Mexican Women, Public Prenatal Care and the Birth Weight Paradox. Rutgers University Press, 2011.

AlyshiaGalvez. Patient Citizens, Immigrant Mothers: Mexican Women, Public Prenatal Care and the Birth Weight Paradox. Rutgers University Press, 2011.

Patient citizens, immigrant mothers: Mexican women, public prenatal care, and the birth-weight paradox

Patient citizens, immigrant mothers: Mexican women, public prenatal care, and the birth-weight paradox

Rejoinder

Rejoinder

Conditioning on Intermediates in Perinatal Epidemiology

It is common practice in perinatal epidemiology to calculate gestational-age-specific or birth-weight-specific associations between an exposure and a perinatal outcome. Gestational age or birth weight, for example, might lie on a pathway from the exposure to the outcome. This practice of conditioning on a potential intermediate has come under critique for various reasons. First, if one is interested in assessing the overall effect of an exposure on an outcome, it is not necessary to stratify, and indeed, it is important not to stratify, on an intermediate. Second, if one does condition on an intermediate, to try to obtain what might conceived of as a "direct effect" of the exposure on the outcome, then various biases and paradoxical results can arise. It is now well documented theoretically and empirically that, when there is an unmeasured common cause of the intermediate and the outcome, associations adjusted for the intermediate are subject to bias. In this paper, we propose 3 approaches to facilitate valid inference when effects conditional on an intermediate are in view. These 3 approaches correspond to (i) conditioning on the predicted risk of the intermediate, (ii) conditioning on the intermediate itself in conjunction with sensitivity analysis, and (iii) conditioning on the subgroup of individuals for whom the intermediate would occur irrespective of the exposure received. The second and third approaches both require sensitivity analysis, and they result in a range of estimates. Each of the 3 approaches can be used to resolve the "birth-weight paradox" that exposures such as maternal smoking seem to have a protective effect among low-birth-weight infants. The various methodologic approaches described in this paper are applicable to a number of similar settings in perinatal epidemiology.

Commentary

In the 1939 Metro-Goldwyn-Mayer film, The Wizard of Oz, the benevolent Wizard is asked to solve the existential dilemmas of the Scarecrow, Tin Man and Cowardly Lion. Seemingly unable to provide them the intangible things that they seek, he instead gives them three mundane items that he happens to have in his bag. The munificence of the Wizard is not cast into doubt by this sleight of hand, however, because the characters discover that the traits they sought were things they had possessed all along. The article by VanderWeele and colleagues1 responds to a crisis in reproductive epidemiology over how to handle measures of maturity and development. Variables such asgestational age and birth weight are ubiquitous in perinatal research, but problematic for all the reasons described in their paper. Analysts want to condition on some measure of maturation to put neonates on an equal developmental footing, but exposures of interest may affect the progress of the pregnancy, making conditional estimates biased as measures of total effect. At the same time, direct effects are generally not identifiable because of unmeasured common causes of the maturity indicator and the outcome, leading to the birth-weight paradox described by the authors. Reproductive epidemiologists therefore look eagerly to methods wizards for a magical solution to their problem: for the brains, heart and courage necessary for their daily travails. What have the methodologists got in their bag?

The epidemiologic paradox of low birth weight in Brazil

OBJECTIVE: To examine whether the low birth weight (LBW) paradox exists in Brazil. METHODS: LBW and cesarean section rates between 1995 and 2007 were estimated based on data from SINASC (Brazilian Live Births Database). Infant mortality rates (IMRs) were obtained using an indirect method that correct for underreporting. Schooling information was obtained from census data. Trends in LBW rate were assessed using joinpoint regression models. The correlations between LBW rate and other indicators were graphically assessed by lowess regression and tested using Spearman's rank correlation. RESULTS: In Brazil, LBW rate trends were non-linear and non-significant: the rate dropped from 7.9% in 1995 to 7.7% in 2000, then increased to 8.2% in 2003 and remained nearly steady thereafter at 8.2% in 2007. However, trends varied among Brazilian regions: there were significant increases in the North from 1999 to 2003 (2.7% per year), and in the South (1.0% per year) and Central-West regions (0.6% per year) from 1995 to 2007. For the entire period studied, higher LBW and lower IMRs were seen in more developed compared to less developed regions. In Brazilian States, in 2005, the higher the IMR rate, the lower the LBW rate (p=0.009); the lower the low schooling rate, the lower the LBW rate (p=0.007); the higher the number of neonatal intensive care beds per 1,000 live births, the higher the LBW rate (p=0.036). CONCLUSIONS: The low birth weight paradox was seen in Brazil. LBW rate is increasing in some Brazilian regions. Regional differences in LBW rate seem to be more associated to availability of perinatal care services than underlying social conditions.

Another Mexican birthweight paradox? The role of residential enclaves and neighborhood poverty in the birthweight of Mexican-origin infants

Examining whether contextual factors influence the birth outcomes of Mexican-origin infants in the US may contribute to assessing rival explanations for the so-called Mexican health paradox. We examined whether birthweight among infants born to Mexican-origin women in the US was associated with Mexican residential enclaves and exposure to neighborhood poverty, and whether these associations were modified by nativity (i.e. mother's place of birth). We calculated metropolitan indices of neighborhood exposure to Mexican-origin population and poverty for the Mexican-origin population, and merged with individual-level, year 2000 natality data (n=490,332). We distinguished between neighborhood exposure to US-born Mexican-origin population (i.e. ethnic enclaves) and neighborhood exposure to foreign-born (i.e. Mexico-born) Mexican-origin population (i.e. immigrant enclaves). We used 2-level hierarchical linear regression models adjusting for individual, metropolitan, and regional covariates and stratified by nativity. We found that living in metropolitan areas with high residential segregation of US-born Mexican-origin residents (i.e. high prevalence of ethnic enclaves) was associated with lower birthweight for infants of US-born Mexican-origin mothers before and after covariate adjustment. When simultaneously adjusting for exposure to ethnic and immigrant enclaves, the latter became positively associated with birthweight and the negative effect of the former increased, among US-born mothers. We found no contextual birthweight associations for mothers born in Mexico in adjusted models. Our findings highlight a differential effect of context by nativity, and the potential health effects of ethnic enclaves, which are possibly a marker of downward assimilation, among US-born Mexican-origin women.

Quantification of collider‐stratification bias and the birthweight paradox

The 'birthweight paradox' describes the phenomenon whereby birthweight-specific mortality curves cross when stratified on other exposures, most notably cigarette smoking. The paradox has been noted widely in the literature and numerous explanations and corrections have been suggested. Recently, causal diagrams have been used to illustrate the possibility for collider-stratification bias in models adjusting for birthweight. When two variables share a common effect, stratification on the variable representing that effect induces a statistical relation between otherwise independent factors. This bias has been proposed to explain the birthweight paradox. Causal diagrams may illustrate sources of bias, but are limited to describing qualitative effects. In this paper, we provide causal diagrams that illustrate the birthweight paradox and use a simulation study to quantify the collider-stratification bias under a range of circumstances. Considered circumstances include exposures with and without direct effects on neonatal mortality, as well as with and without indirect effects acting through birthweight on neonatal mortality. The results of these simulations illustrate that when the birthweight-mortality relation is subject to substantial uncontrolled confounding, the bias on estimates of effect adjusted for birthweight may be sufficient to yield opposite causal conclusions, i.e. a factor that poses increased risk appears protective. Effects on stratum-specific birthweight-mortality curves were considered to illustrate the connection between collider-stratification bias and the crossing of the curves. The simulations demonstrate the conditions necessary to give rise to empirical evidence of the paradox.

Z‐scores and the birthweight paradox

Investigators have long puzzled over the observation that low-birthweight babies of smokers tend to fare better than low-birthweight babies of non-smokers. Similar observations have been made with regard to factors other than smoking status, including socio-economic status, race and parity. Use of standardised birthweights, or birthweight z-scores, has been proposed as an approach to resolve the crossing of the curves that is the hallmark of the so-called birthweight paradox. In this paper, we utilise directed acyclic graphs, analytical proofs and an extensive simulation study to consider the use of z-scores of birthweight and their effect on statistical analysis. We illustrate the causal questions implied by inclusion of birthweight in statistical models, and illustrate the utility of models that include birthweight or z-scores to address those questions. Both analytically and through a simulation study we show that neither birthweight nor z-score adjustment may be used for effect decomposition. The z-score approach yields an unbiased estimate of the total effect, even when collider-stratification would adversely impact estimates from birthweight-adjusted models; however, the total effect could have been estimated more directly with an unadjusted model. The use of z-scores does not add additional information beyond the use of unadjusted models. Thus, the ability of z-scores to successfully resolve the paradoxical crossing of mortality curves is due to an alteration in the causal parameter being estimated (total effect), rather than adjustment for confounding or effect decomposition or other factors.

Z -scores and the birthweight paradox

Z -scores and the birthweight paradox

Baixo peso ao nascer no Brasil de acordo com as informações sobre nascidos vivos do Ministério da Saúde, 2005

Although the Brazilian Information System on Live Births (SINASC) has expanded its coverage and improved its data quality since the system was implemented, the live birth count in Brazil is still not complete. The current study analyzes inequalities in low birth weight in Brazil in 2005, based on geographic factors, municipality of residence, maternal schooling, and prenatal care. An analysis of all live births showed a low birth weight paradox, namely high percentages in areas with the highest socioeconomic development. The main explanation for these paradoxical findings involves shorter neonatal survival and inadequate recording of premature births in poorer municipalities. Considering at-term live births from non-multiple gestations, inequalities were found in maternal schooling. Comprehensive, quality prenatal care could have a greater impact by reducing negative outcomes of gestation and decreasing the socioeconomic inequalities of perinatal health in Brazil.

From causal diagrams to birth weight-specific curves of infant mortality

This report explores the low birth weight paradox using two graphical approaches: causal directed acyclic graphs (DAGs), and the empirical curves of the birth weight distribution and birth weight-specific mortality. The birth weight curves are able to represent the associations quantitatively, while the corresponding causal DAGs provide a set of plausible explanations for the findings. Taken together, these two approaches can facilitate discussion of underlying biological mechanisms.

Air Pollution and Birth Weight in Chuvashia: Low Birth Weight Paradox

Air Pollution and Birth Weight in Chuvashia: Low Birth Weight Paradox