BACKGROUND: The advent of fertility treatments has led to an increase in the rate of multiple births in the United States. However, the trends in and magnitude of the contribution of fertility treatments to the increase are uncertain. METHODS: We derived the rates of multiple births after natural conception from data on distributions of all births from 1962 through 1966 (before fertility treatments were available). Publicly available data on births from 1971 through 2011 were used to determine national multiple birth rates, and data on in vitro fertilization (IVF) from 1997 through 2011 were used to estimate the annual proportion of multiple births that were attributable to IVF and to non-IVF fertility treatments, after adjustment for maternal age. Trends in multiple births were examined starting from 1998, the year when clinical practice guidelines for IVF were developed with an aim toward reducing the incidence of multiple births. RESULTS: We estimated that by 2011, a total of 36% of twin births and 77% of triplet and higher-order births resulted from conception assisted by fertility treatments. The observed incidence of twin births increased by a factor of 1.9 from 1971 to 2009. The incidence of triplet and higher-order births increased by a factor of 6.7 from 1971 to 1998 and decreased by 29% from 1998 to 2011. This decrease coincided with a 70% reduction in the transfer of three or more embryos during IVF (P<0.001) and a 33% decrease in the proportion of triplet and higher-order births attributable to IVF (P<0.001). CONCLUSIONS: Over the past four decades, the increased use of fertility treatments in the United States has been associated with a substantial rise in the rate of multiple births. The rate of triplet and higher-order births has declined over the past decade in the context of a reduction in the transfer of three or more embryos during IVF.
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