Birthweight and perinatal mortality: I. On the frequency distribution of birthweight.

Perinatal mortality is closely related to birthweight. Hence the study of perinatal mortality requires a sound understanding of the influence of birthweight on perinatal mortality. This paper discusses one aspect of this problem--the frequency distribution of birthweight. This distribution is essentially Gaussian but with additional births in the lower tail. It can therefore be divided into two components--a predominant (Gaussian) distribution and a residual distribution. The complete distribution can be summarized by three parameters: the mean and the standard deviation of the predominant distribution, and the proportion of births in the residual distribution. This paper shows that the predominant distribution is composed largely of term births, while the residual distribution is composed almost entirely of small preterm births. It also shows that the three parameters together help to explain the apparent paradox that male infants suffer a higher perinatal mortality than females despite there being fewer light male births.