The number of twins has doubled and the rate of twin births has risen by more than three-fourths over the three decades 1980–2009. Increases in twin birth rates averaged more than 2 percent annually from 1980 to 2004, but the pace of increase slowed to less than 1 percent from 2005 to 2009. The increase in twinning over the three decades has been widespread, occurring across age and race and Hispanic origin groups, and in all states within the United States. The largest increases have been among non-Hispanic white women and mothers aged 30 and over. Older maternal age accounts for about one-third of the growth in the twinning rate over this period. The increased availability and use of infertility treatments likely explains much of the remainder of the rise (9,10). Similar increasing trends in multiple births associated with both maternal age and infertility therapies have been observed in Western Europe and other countries during the 1980s and 1990s (10,12). The study of multiple births is important because of their elevated health risks and accompanying greater health care costs (6). The rise in twinning has influenced the upward trend in key infant health indicators such as preterm and low birthweight rates during the 1980s and 1990s (8). An estimated additional 865,000 twins were born in the United States over the study period due to rising rates; more than one-half of these infants were low birthweight, and 1 in 10 were very low birthweight (3,4,13). New information soon to be available from the 2003 revision of the U.S. Standard Certificate of Live Birth and the U.S. Standard Report of Fetal Death on topics such as use of infertility treatment and admission to a neonatal intensive care unit, will further expand understanding of the causes and risks of multiple-gestation pregnancies.
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