An Econometric Model of Birth Inputs and Outputs

This study offers a simultaneous equations model of the birth process with seven endogenous variables: four birth inputs [maternal smoking (S), maternal drinking (D), first trimester prenatal care (PC), and maternal weight gain (WG)], and three birth outputs [gestational age (G), birth length (BL), and birth weight (BW)]. Our analysis conditions on twenty-four exogenous variables. The data are taken from the NLSY. Separate analyses are performed on five different groups: Whites (both the Main and the Supplemental samples), Blacks, Hispanics, and Native Americans. Across all groups, we find sizeable correlation between the disturbances in the four input and three output equations and among output disturbances. Ceteris Paribus, the effect of maternal smoking on BL and BW is negative, the effect of weight gain on BL and BW is positive, long gestation has a favorable effect on both BL and BW, a male infant is longer and heavier than a female infant, and maternal height and weight have a positive effect on BL and BW, respectively. Surprisingly, we find that the widely-cited group differences in birth outputs can be accommodated in our framework with simple group dummies. Our framework also sheds some light on the High/Low Risk Birth Weight Puzzle discussed in the literature. Finally, our results are robust with respect to different model and prior specifications.