A criterion for a standardized definition of low birthweight.

A criterion for the definition of low birthweight (LBW) which is tailored to each population group is presented. The suggested criterion is based on an assumption according to which the distribution of birthweight is actually a mixture of the following two components: the predominant (including about 95% of the newborns and is normally distributed) and the residual (assumed to be mainly composed of newborns under extreme risk for perinatal mortality). The LBW category is defined to include a small but equal (for all groups) percentage of the predominant component. Thus, population groups which differ with respect to their specific predominant birthweight distribution are comparable with respect to the perinatal mortality risk. Application of the suggested definition is demonstrated using data from two ethnic groups in Israel. Results show a smaller percentage of female LBW than males in both population groups when using the suggested criterion. The reverse is observed (i.e. more male than female LBW), using the standard criterion (less than 2500 gm). The suggested criterion is shown to reduce the sex and ethnic differences in perinatal mortality among the respective LBW categories.