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Table 2 Temporal trends in rates of twin, and triplet and higher-order (triplet+) multiple live births in Canada (excluding Ontario) and the United States, 1991–2009

From: Temporal trends in the frequency of twins and higher-order multiple births in Canada and the United States

Time period Canada (excluding Ontario) United States
  Change in rate between first and last year of interval P-value* Change in rate between first and last year of interval P-value*
  Absolute change in rate (95% CI)† Relative change in rate (95% CI)   Absolute change in rate (95% CI)† Relative change in rate (95% CI)  
Twins
1991–2004 8.3 (7.4 to 9.2) 41% (36% to 47%) <0.0001 9.1 (8.9 to 9.3) 39% (38% to 41%) <0.0001
2004–2007 1.2 (0.2 to 2.2) 4% (1% to 8%) 0.0005 0 (-0.2 to 0.3) 0% (-1% to 1%) 0.81
2007–2009 1.9 (0.9 to 2.9) 6% (3% to 10%) 0.0001 1.0 (0.8 to 1.3) 3% (2% to 4%) <0.0001
Triplet+       
1991–1998 45.0 (29.0 to 61.1) 86% (50% to 132%) <0.0001 112.1 (106.9 to 117.2) 138% (128% to 148%) <0.0001
1998–2003 13.9 (-5.8 to 33.5) 14% (-5% to 38%) 0.04 -6.1 (-12.1 to -0.1) -3% (-6% to 0%) 0.13
2003–2007 -33.5 (-51.9 to -15.1) -30% (-43% to -15%) <0.0001 -38.5 (-44.0 to -32.9) -21% (-23% to -18%) <0.0001
2007–2009 5.9 (-10.3 to 22.1) 6% (-12% to 32%) 0.49 4.6 (-0.7 to 9.8) 3% (0% to 7%) 0.09
  1. * Two-sided Cochrane-Armitage test for linear trend. † Absolute change in rate per 1,000 live births for twins and per 100,000 live births for triplet+.