More than a third of twin births and more than three-quarters of triplets or higher-order births in the United States are the result of assisted reproductive technologies, said a new finding published Wednesday.

The researchers from Brown University gathered data on multiple births from 1962 to 1966 before any medical fertility treatments were available and from 1971 through 2011, when data on in vitro fertilization (IVF) procedures has been available.

They found that the contribution of fertility treatments over the past 40 years is clear: Between 1971 and 2011, the proportion of U.S. multiple births rose to 3.5 percent from 1.8 percent.

Even after adjusting for maternal age, the rate of twin births rose 1.6 times between 1971 and 2009, the researchers said.

However, the proportion of triplets or more related to fertility treatments has actually dropped from a peak of 84 percent in 1998 after IVF guidelines discouraging implantation of three or more embryos took effect that year, researchers at Brown University said.

Some mothers and couples may hope for twins through fertility treatments, but more often multiple births are not desired, said senior author Eli Adashi, professor of obstetrics and gynecology at Brown University.

“We do have a real problem with way too many multiple births in the United States with consequences to both mothers and babies,” Adashi said in a statement.

While multiple births from IVF are a direct result of the number of embryos that are fertilized and intentionally implanted, non-IVF therapies involve medications that stimulate ovulation and follicle growth in ways that cannot be precisely predicted or controlled, the researchers said.

“IVF is moving, in a sense, in the right direction and cleaning up its act, whereas the non-IVF technologies are at a minimum holding their own and possibly getting worse,” Adashi said. “From a policy point of view what that means is that (we) need to focus on the non-IVF technologies, which really hasn’t been done in a concerted way because they weren’t considered all that relevant to this mix.”

The findings were published in the U.S. journal New England Journal of Medicine.

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Editor: Yamei Wang