Natural selection associated with birth weight. III. Changes over the last twenty years

Differential stillbirth rates as a function of birth-weight have been studied in all single births in Italy in the years, 1954, 1961, 1967 and 1974. In the course of the twenty year interval of observation the following changes were found: (i) a progressive equalization of mortality in birth-weight classes near the mean; (ii) a reduction of the directional component of selection which is parallel to the increase of mean birth-weight (in the case of 8 months of pregnancy); (iii) a reduction of the stabilizing component of selection which is parallel to the decrease of birth-weight variance (in the case of 9 months of pregnancy); (iv) a reduction of selection intensity, while selective mortality remains more or less unchanged. The modifications of natural selection associated with birth-weight as consequence of health care progress in this population are also discussed.