The authors examined secular trends in birth weight for a geographically defined population over 40 years, controlling for migration effects. The study was an analysis first of all Illinois births between 1950 and 1990 and second of a subset of births for which two succeeding generations were born in the state. For the latter analysis, the authors created a transgenerational birth file by linking infant birth records to the birth records of their parents. Shifts toward bigger babies were observed in both data sets. For black births, the shift was larger in the transgenerational file; but for white infants, similar magnitude shifts were observed in the two files. In both analyses, there were larger birth weight shifts for whites than for blacks. Mean birth weight increases within families ranged from 33 g (black male infants compared with their fathers) to 74 g (white female infants and their mothers). The rate of births at very low birth weight (< 1,500 g) decreased by 6% in the white population but increased by 56% in blacks. Results presented in this study demonstrate that even when migratory effects are eliminated, a secular increase in birth weight is observed. Moreover, the left tall of the birth weight distribution does not always follow the same temporal trend observed for the mode.