The changing pattern of multiple births in the United States: maternal and infant characteristics, 1973 and 1990.
To evaluate changes in the incidence and proportion of live births by plurality in the United States between 1960 and 1990, and to compare the distribution of singleton, twin, and triplet and higher-order births (triplet+) by maternal and infant characteristics for 1973 and 1990. Vital statistics data were used to conduct a population-based analysis of all live births from 1960-1990 and to compare changes in the incidence and outcomes of live births between 1973 and 1990 by maternal and infant characteristics. Between 1960 and 1973, the number of twin births paralleled that of singletons, whereas the number of triplet+ births remained fairly constant. Between 1973 and 1990, twin births increased at twice the rate of singletons (65 versus 32%), and triplet+ births increased at seven times the rate of singletons (221 versus 32%). The resulting frequencies of occurrence changed from one in 55 to one in 43 births for twins and from one in 3323 to one in 1341 for triplet+ births. In 1990, preterm births accounted for 9.7% of singleton births, compared to 47.9% of twin births and 87.8% of triplet+ births. Because of the greater frequency of twin and triplet+ births in 1990, the observed number of very low and low birth weight infants was 24.2% greater among twin births and 142.3% greater among triplet+ births than would be expected if the 1973 ratios to singleton births had remained constant. The rise in multiple births, with the associated greater risks of prematurity and low birth weight, is of national importance. Comprehensive and aggressive prenatal care to assure the best outcomes should be the goal for clinicians caring for these women.