Non-linear association of low birthweight with risk factors including women’s BMI: Evidence from an international comparison

Abstract Background It is widely known that, in addition to physical characteristics such as parents’ height and weight, social attributes such as parents’ education, income level, and employment status have a significant impact on birthweight. However, these results were obtained using data from individual countries, and there were no country-level comparisons of the factors identified as influencing birthweight. Methods Using panel data on low-birthweight fertility rates for 143 countries over the period of 2000–2015, we used five factors (women’s body mass index [BMI], real gross domestic product [GDP] per capita, women’s employment status, healthcare level, and adolescent childbearing) in relation to countries’ low-birthweight rates to determine how these factors relate to each country's low-birthweight rate and estimate a fixed-effects model. Considering the possibility that these five factors are non-linear rather than linear, we estimated a quadratic function. We conducted a detailed analysis of women’s BMI and real GDP per capita. Furthermore, we considered the 2008 global financial crisis as an exogenous natural experiment for the low-birthweight rate and conducted a difference-indifferences (DID) analysis to confirm the possibility that the correlation between women’s employment rate and low-birthweight rate is a causal relationship. Results All five factors were identified as important risk factors. Of the five, all but adolescent childbearing were found to have a non-linear rather than a monotonous linear relationship with low birthweight. The low-birthweight rate improved sharply with improvement in women's average BMI below 28. Furthermore, the results of the DID analysis suggest a causal relationship between women's employment and low-birthweight rates. Conclusions These results provide a useful policy tool for achieving the goal of the 65th World Health Assembly to “reduce the incidence of low birthweight.” In particular, improvements in women’s average BMI, real GDP per capita, and women’s employment rates in low-income countries are expected to reduce the incidence of low birthweight.