A comparison of recent trends in infant mortality among twins and singletons.

Overall infant mortality rates have steadily declined in recent years. The goal of this study was to examine whether recent declines in infant mortality were similar for twins and singletons, and to assess the impact of differing birthweight distributions on these relationships. Linked birth and infant death records for 1985-86 and 1995-96 were used to calculate infant mortality rates for twins and singletons for the two time periods. Bootstrap simulations were used to estimate rates of decrease between the two time periods and to determine whether these rates differed between twins and singletons. Between 1985-86 and 1995-96, infant mortality among twins declined significantly faster than among singletons (36% vs. 29%, P < 0.05). This difference was true for both black and white infants (black: 28% for twins vs. 22% for singletons; white: 38% for twins vs. 31% for singletons). Within birthweight categories, infant mortality declined more rapidly among twins than among singletons, although differences were not always significant. Factors and circumstances that contributed to the infant mortality decline in the United States have benefited twins to a greater extent than singletons.