During 1980-1994, the number of twin births in the United States increased by 42%, from 68,339 to 97,064, and the twin birth rate (i.e., the number of twin births to total live births) increased 30%, from 18.9 to 24.6 per 1000 live births. These increases are important because the risks for preterm birth, low birthweight (LBW), long-term disability, and early death are greater for twins than for singletons (1; CDC, unpublished data, 1991). to estimate state-specific rates of twin births, CDC analyzed data from the U.S. certificates of birth for 1992-1994. This report presents the findings of this analysis of these data, which indicate that state-specific rates of twin births varied substantially, and the variations reflect factors other than state-specific differences in maternal age distributions.