Understanding infant health production where mother preconception BMI and gestational weight gain are two key inputs is of great importance to policymakers who strive to improve population health. On one hand, both the rise in preconception obesity (or overweight) among women of reproductive age and the high prevalence of gestational excessive weight gain among pregnant women can lead to larger risks of having high birth weight (HBW) babies. On the other hand, inadequate maternal energy intake before or during pregnancy (preconception underweight or inadequate gestational weight gain) are associated with a higher chance of delivering low birth weight (LBW) infants. Both HBW and LBW are costly birth outcomes which have lasting adverse effects on a variety of lifetime outcomes. Therefore, if the associations above turn out be to causal, policy interventions that help women to get healthy normal weight and gain appropriate weight during pregnancy can generate large intergenerational benefits.  

The past studies on this topic have provided mix evidence. In particular, they have one or more of the following limitations: applying small selective samples, lack of a key mother body mass measure (preconception BMI or gestational weight gain) and important control variables, focusing on one end of the birth weight distribution, or ignoring the unobserved mother heterogeneity.  This paper uses a large dataset of sibling births to study the effects of pre-pregnancy BMI and gestational weight gain on infant health.  Compared with the past studies, this research is not subject to any of the aforementioned limitations.

From the baseline regressions with mother fixed effects, there are two primary findings. One, preconception obesity or overweight significantly increases the risks of having HBW infants while preconception underweight leads to a higher probability of delivering LBW babies. Those estimates capture the direct effects of preconception BMI categories on infant health, because their indirect effects which operate through gestational weight gain are not identified given that gestational weight gain has been controlled. Two, women with excessive weight gain during pregnancy are more likely to have HBW babies, while those with inadequate weight gain have higher risks of delivering LBW infants. The results are consistent with the benchmark estimates when the analysis is extended to the following cases: using an earlier IOM guideline of gestational weight gain, stratifying the sample by state or number of births, applying more homogenous subsamples, using additional interactive terms or nonlinear specifications. Overall, this paper suggests there are two approaches for policymakers to improve infant health: first, reducing the prevalence of unhealthy weight among women of reproductive age through weight control interventions; second, helping overweight or obese (underweight) women to gain appropriate weight during pregnancy through programs such as Medicaid or WIC.