Oct. 28 (HealthDay News) -- Women undergoing a second round of in-vitro fertilization should get one embryo instead of two, suggests new Swedish research that found the first approach is almost as successful as the second and greatly reduces the risk of multiple births.

A previous study produced the same result, but this latest report examined the women for a longer period of time, through more embryo implantations.

The findings "should provide comfort for those who want to perform single-embryo transfers that the pregnancy rates are equivalent," said Dr. William E. Gibbons, president of the American Society for Reproductive Medicine and a professor at Baylor College of Medicine in Houston.

Gibbons said debate over the number of embryos to implant began about a decade ago, when in-vitro fertilization (IVF) began to be more successful.

"Since only a fraction of eggs released by a woman are capable of producing a baby, the pregnancy rate is higher when more than one embryo is put back in," he said. "However, in the late '90s the standard number of embryos replaced at that time resulted in an increasing rate of triplets and quadruplets."

Multiple births can lead to a variety of medical complications that can put the lives of some babies at risk.

In the new study, the Swedish researchers examined the records of 661 women who first underwent implantation with a fresh embryo, followed by implantation with one or two frozen embryos if the first attempt was unsuccessful. If those attempts failed, they went through more attempts; the scientists followed them for up to four more tries.

The findings appear in the Oct. 29 issue of the New England Journal of Medicine.

Forty-four percent of those who received one embryo the first time got pregnant and gave birth to live babies; 51 percent of those who got two embryos did.

But only 2.3 percent of those in the first group had multiple births, compared to 27.5 percent in the second group. Those in the first group were also much less likely to have babies born before 37 weeks of gestation, 11.8 percent vs. 25.5 percent for the two-embryo group.

So why not always implant one embryo and then another if necessary? Because it can be more expensive to try multiple times instead of boosting the odds at the start, and "the stress and disappointment of a failed cycle is hard to put a value on," noted Dr. Laurel Stadtmauer, an associate professor of obstetrics and gynecology at the Jones Institute for Reproductive Medicine in Norfolk, Va.

The economic wrinkle is a real one. In Sweden, Gibbons explained, IVF is covered by national insurance. That's not the case in the United States.

"For American couples," he said, "putting two embryos back has a higher pregnancy rate, and twins mean that they can have their family all at once and don't have to pay for a second child."

SOURCES: William E. Gibbons, M.D., president, American Society for Reproductive Medicine, and professor, Baylor College of Medicine, Houston; Laurel Stadtmauer, M.D., Ph.D., associate professor, obstetrics and gynecology, Jones Institute for Reproductive Medicine, Norfolk, Va.; Oct. 29, 2009, New England Journal of Medicine