Low birthweight for gestation and short gestation for birthweight are included as new risk factors for GBD 2016. The estimation has been parameterised to be polytomous by 500 g and 2 week categories. Low birthweight and gestational age are highly correlated risks and they are estimated in a completely interdependent manner. For each univariate analysis, identification of TMREL and calculation of PAFs is contingent on the other dimension. In other words, we found the lowest risk birthweight category for each 2 week gestational age band and, correspondingly, the lowest risk gestational age for each 500 g birthweight band. RRs were then estimated for each 500 g per 2 week bin. Exposure for each bin was estimated in three steps. First, we estimated by generating ensemble distribution estimates using modelled mean and categorical prevalence estimates for each of birthweight (mean, % <2500 g) and gestational age (mean, % <37 weeks, % <28 weeks) for each location, year, and sex. Second, we evaluated all microdata where both gestational age and birthweight were available and found a high degree of consistency in the
