The reunification in 1990 of the German Democratic Republic (GDR) and the Federal Republic of Germany (FRG) produced profound social transformations. Changes in the distribution of low birthweight (LBW, < 2500 g) in the two parts of the country between 1990 and 1992 have been studied by analysing vital statistics for the period. In absolute numbers, livebirths in the former FRG remained stable, while those in the former GDR declined by 51%. Numbers of LBW livebirths increased slightly in the former FRG and decreased in the former GDR; those in the category between 500 and 999 g remained stable in the former GDR and increased in the former FRG. However in terms of proportion, livebirths in this category doubled in the former GDR. Migration rates for the same period showed a shifting population from East to West particularly of young people, and maternal age-specific numbers of livebirths decreased in both countries. Psychosocial stress may have contributed to the rise in ELBW, but it is also possible that the improvement and sharing of perinatal management strategies may have led to increased survival of babies < 1000 g. Most importantly, the observed rise in the proportion of ELBW births (except those < 500 g) could be a result of the introduction of the more comprehensive definition of livebirth into the former GDR.