Birth Weight Paradox Research Articles

Invited Commentary: The Perils of Birth Weight--A Lesson from Directed Acyclic Graphs

The strong association of birth weight with infant mortality is complicated by a paradoxical finding: Small babies in high-risk populations usually have lower risk than small babies in low-risk populations. In this issue of the Journal, Hernández-Díaz et al. (Am J Epidemiol 2006;164:1115-20) address this "birth weight paradox" using directed acyclic graphs (DAGs). They conclude that the paradox is the result of bias created by adjustment for a factor (birth weight) that is affected by the exposure of interest and at the same time shares causes with the outcome (mortality). While this bias has been discussed before, the DAGs presented by Hernández-Díaz et al. provide more firmly grounded criticism. The DAGs demonstrate (as do many other examples) that seemingly reasonable adjustments can distort epidemiologic results. In this commentary, the birth weight paradox is shown to be an illustration of Simpson's Paradox. It is possible for a factor to be protective within every stratum of a variable and yet be damaging overall. Questions remain as to the causal role of birth weight.

Which factors could explain the low birth weight paradox?

Low birth weight children are unusual among well-off families. However, in Brazil, low birth weight rate was higher in a more developed city than in a less developed one. The study objective was to find out the reasons to explain this paradox. A study was carried out in two municipalities, Ribeirão Preto (Southeastern Brazil) and São Luís (Northeastern Brazil), which low birth weight rates were 10.7% and 7.6% respectively. Data from two birth cohorts were analyzed: 2,839 newborns in Ribeirão Preto in 1994 and 2,439 births in São Luís in 1997-1998. Multiple logistic regression analysis was performed, adjusted for confounders. Low birth weight risk factors in São Luís were primiparity, maternal smoking and maternal age less than 18 years. In Ribeirão Preto, the associated variables were family income between one and three minimum wages, maternal age less than 18 and equal to or more than 35 years, maternal smoking and cesarean section. In a combined model including both cohorts, Ribeirão Preto presented a 45% higher risk of low birth weight than São Luís. When adjusted for maternal smoking habit, the excess risk for low birth weight in Ribeirão Preto compared to São Luís was reduced by 49%, but the confidence interval was marginally significant. Differences in cesarean section rates between both cities contributed to partially explain the paradox. Maternal smoking was the most important risk factor for explaining the difference in low birth weight between both cities. The other factors contributed little to explain the difference in low birth weight rates.

Education, infant mortality, and low birth weight in Sweden 1973—1990: Emergence of the low birth weight paradox

Educational differences in infant mortality, birth weight, and birth weight-specific infant mortality in Sweden were analysed. The "low birth weight paradox", where low birth weight infants have a lower mortality risk if born to women of lower rather than higher social strata, was addressed. The study includes about a million single births 1973-90 to women born 1946-60. There were 6,544 infant deaths and 35,334 low birth weight infants. Analysis conducted on six-year time periods was restricted to 652,859 births to women aged 25-32 at the time of delivery. Odds ratios and 95% CI were estimated by logistic regression. Birth weight-specific infant mortality rates were calculated by education. Infants of women with low/low intermediate education had significantly higher odds ratios than those of highly educated women of being of low birth weight or of dying. If one compares only the infants of women with low and high education, these differences were accentuated over time. The low birth weight paradox appears over time. The widening differences in infant mortality and low birth weight over time may be caused by the decrease in women with low education, signifying increased selection and growing social disadvantage in this group. The emergence of the low birth weight paradox suggests that the distribution of causes of low birth weight differs between educational groups, and further that these causes are differently related to infant mortality. To disentangle these two groups of causes and their effects on infant mortality seems highly relevant.

Explaining the birth weight paradox: Latina immigrants' perceptions of resilience and risk.

In the US, prenatal care is positively associated with improved birth outcomes. However, among Mexican-born Latinos, rates of low birth weight are lower than those of US-born counterparts despite the fact that recently arrived Latino immigrants are less likely to have received adequate prenatal care. The "birth weight paradox," identified through analysis of the HHANES, appears to hold constant across variations in age, marital status or educational attainment. The authors explore Latina immigrant's perceptions of resilience factors related to these better birth outcomes through focused group interviews, photonarratives, and documentation of local kin networks. The women's responses are grouped into five resilience factors and one risk complex that have the potential to further explain the HHANES findings. Women's responses, the stories of their photographs, and kin networks are presented. Knowledge of these protective and risk factors can be useful to health professionals and Latino advocacy groups in the design of community-based interventions that protect health status and promote the practice of protective health behaviors within immigrant families and communities.