Multiple Births on the Decline in the United States

Researchers at the US Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA) derived the rates of multiple births from natural conception from the distributions of all births from 1962 to 1966, before fertility treatments were available. Publicly available data on births from 1971-2011 were used to determine national multiple birth rates, and data on in vitro fertilization (IVF) from 1997–2011 were used to estimate the annual proportion of multiple births attributable to IVF and to non-IVF fertility treatments, after adjustment for maternal age. The researchers then examined trends in multiple births starting from 1998, the year when clinical practice guidelines for IVF were developed with an aim toward reducing the incidence of multiple births.

The results showed that the observed incidence of twin births increased by a factor of 1.9 from 1971 to 2009, from 1.8% to 3.5%, and the rate of twin births grew by a factor of 1.9. The incidence of triplet and higher-order births increased by a factor of 6.7 from 1971 to 1998, and then decreased by 29% from 1998 to 2011. This decrease coincided with a 70% reduction in the transfer of three or more embryos during IVF, and a 33% decrease in the proportion of triplet and higher-order births attributable to IVF. The researchers then considered maternal age, with older mothers being more likely to have multiple births.

The analysis revealed that between 1971 and 2011, among women under 30, the frequency of twin births fell by 39%, while multiple births fell by 60%. In comparison, women older than 30 had double the rate of multiple births over that time period, increasing from 24% to 54%. After adjustment for maternal age, twin births rose by a factor of 1.6 and high-order multiple births by a factor of 4.8. The researchers added that by 2011, a total of 36% of twin births and 77% of triplet and higher-order births resulted from conception assisted by IVF. The study was published in the December 5, 2013, issue of the New England Journal of Medicine.

“The forces underlying the decline in the number of embryos transferred probably include professional societies, which have repeatedly revised practice guidelines to include recommendations for lowering the number of embryos transferred during IVF,” concluded lead author Aniket Kulkarni, MBBS, and colleagues. “Increased acceptance of elective single-embryo transfers is likely to require a shift in clinical emphasis from a cross-sectional, cycle-centered mindset to a longitudinal patient-centered outlook.”

Related Links:
US Centers for Disease Control and Prevention