Birthweight and perinatal mortality: III. Towards a new method of analysis.

Perinatal mortality is closely related to birthweight. We propose a model that summarizes this relationship and provides a basis for the analysis of perinatal mortality. The components of this model are the frequency distribution of birthweight and the curve of weight-specific mortality. Taken together, these two curves completely describe perinatal mortality for a given population. The perinatal mortality of two populations can be meaningfully compared by plotting each weight-specific mortality curve relative to its own birthweight distribution. By this means, the excess mortality in one population can usually be expressed as the sum of two excess mortalities--one that occurs uniformly over the whole birthweight distribution, the other due to an increased number of small births. To illustrate this method, we analyse differences between white and black infants. We find that the excess mortality of black infants is chiefly due to an excess of small black births, but also to higher mortality over all (adjusted) birthweights. In contrast to other methods of analysis, the proposed method does not assume a priori that a difference in the mean birthweight of two populations is the cause of any difference in perinatal mortality. Furthermore, the proposed method is unbiased; in particular, it is preferable to direct or indirect standardization for birthweight, previously shown to be biased.