The objective of this study was to determine the birthweight and gestational age associated with the lowest fetal mortality for twins and triplets. The study design was a population-based analysis of all live births and fetal deaths in the US between 1983-88. Fetal mortality was compared by categories of birthweight and gestational age, for twins and triplets versus singletons, and within each plurality by the lowest rate compared to all other rates as relative risks +/- 95% Cls. The overall versus lowest fetal death rate per 1,000 conceptions for singletons was 4.3 versus 0.9 at 3700-4000 g and 40-41 weeks; for twins, 15.5 versus 3.3 at 2500-2800 g and 36-37 weeks; and for triplets, 21.0 versus 5.2 at 1900-2200 g at 34-35 weeks. Beyond these plurality-specific lowest ranges, the risk of fetal death increased, more for twins than singletons, and most for triplets. To conclude, fetal death rates can be reduced by 75-80% with attainment of birthweight and gestational age within a plurality-specific ideal range.