In Massachusetts during 1989-1996, perinatal health indicators such as infant mortality, teen birth rate, and maternal smoking during pregnancy decreased steadily; however, low birthweight (LBW) (i.e., <2500 g [<5 lbs, 8 oz]) rates increased consistently. During this same period, the multiple-birth rate (i.e., number of twins and higher order multiple births per 100 live births) increased from 2.5% in 1989 to 3.5% in 1996. Massachusetts has the highest multiple-birth rate in the United States. Multiple births are more likely to result in LBW infants. To determine the effect of changes in the rate of multiple births on LBW rates and to characterize women who have multiple births, the Massachusetts Department of Public Health examined data on births in Massachusetts during 1989-1996. This report summarizes the results of this analysis, which indicate that the increase in LBW rates in Massachusetts was associated with changes in the rate of multiple births, especially among older, better educated women.