Fertility technology in the United States has a huge influence on the frequency of twins, triplets, and other multiple births, according to new estimates published Dec. 5 in the New England Journal of Medicine. Dr. Eli Y. Adashi, professor of obstetrics and gynecology（妇产科） at Brown University, and his colleagues calculated that more than a third of twin births and more than three-quarters of triplets or higher-order births in the United States in 2011 were the result of fertility treatments. The proportion of triplets or more related to medical assistance has actually dropped from a peak of 84 percent in 1998 after in vitro fertilization (IVF) guidelines discouraging implantation of three or more embryos took effect that year, the study reports. IVF has also improved enough that single embryo transfers now often succeed in producing healthy pregnancies. But in the meantime, non-IVF fertility treatments such as ovarian stimulation and ovulation induction -- for instance, with the drug clomiphene citrate -- have increased to become the predominant（主要的） source of medically assisted multiple births in the country while IVF is increasingly producing twins.

Some mothers and couples may hope for twins through fertility treatments, Adashi said, but more often multiple births are not desired. In those cases, he said, the new parents and children incur unwarranted medical risk and long-term financial costs that doctors should strive to prevent.

"We do have a real problem with way too many multiple births in the United States with consequences to both mothers and babies," said Adashi, the study's senior author and former dean of the Warren Alpert Medical School of Brown University. "It's an unintended consequence of otherwise well intentioned and remarkable technology."

To arrive at their estimates, the team, including lead author Aniket Kulkarni of the U.S. Centers for Disease Control and Prevention, gathered data on multiple births from 1962 to 1966 (before any medical fertility treatments were available) and from 1971 through 2011. Data on IVF procedures has been available since 1997, but no data is available that directly reflects the contribution of non-IVF procedures to rates of multiple births.

The team therefore estimated the role of non-IVF technologies by subtracting the multiple births arising from IVF from the overall number of multiple births, while also accounting for the impact of maternal age（孕妇年龄） on birth plurality. The data from the 1960s, meanwhile, provided a statistical baseline for natural multiple birth rates without medical intervention that the team also used in their estimates.