Additional file 1.
Figure 1. Temporal trends in rates of twin live births (upper panel) and triplet and higher-order (triplet+) multiple live births (lower panel) in Canada (excluding Ontario), Ontario and the United States, 1991–2009. Plots depict observed rates of twins (per 1,000 live births) and 3-year moving averages of observed rates for triplet+ (per 100,000 live births).
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Fell and Joseph BMC Pregnancy and Childbirth 2012 12:103 doi:10.1186/1471-2393-12-103