By Genevra Pittman

NEW YORK (Reuters Health) - A new study from an Iowa fertility center found that implanting women with one embryo during in vitro fertilization (IVF) didn't lower their chances of having a baby, but it did mean fewer women gave birth to twins.

IVF has traditionally involved implanting multiple embryos, which is known to increase by many times a woman's chances of having twins. Pregnancy with multiples raises certain risks to both moms and babies, including diabetes during pregnancy, premature birth and cerebral palsy.

Dr. Jessica Kresowik from the University of Iowa in Iowa City said she and her colleagues felt that twin rates at their clinic were too high -- so they began requiring that younger women with a good chance of getting pregnant only have one embryo implanted.

The goal was to "increase the outcome that we want, of one healthy baby at a time," she told Reuters Health.

Babies conceived through IVF account for just one percent of U.S. births each year, but IVF is responsible for 17 percent of twins, Kresowik and her colleagues write in the journal Fertility and Sterility.

According to the American Society for Reproductive Medicine, no more than two embryos should be implanted in women under 35 -- and doctors should consider using just one.

But in the U.S., those guidelines have been "slowly adopted," the researchers note, with many clinics still implanting multiple embryos in younger, healthy women.

"In the past, you did need to implant multiple embryos in order to maintain those pregnancy rates," Kresowik said, adding that before technology improved, doctors might use six embryos at once, hoping that one would result in a pregnancy.

Now, she said, more centers are offering single embryos as an option to patients.

Using a single embryo for all women under 38 who were getting their first round of IVF and had other signs predicting a good outcome became the policy at Kresowik's fertility clinic in 2004.

The researchers collected data on IVF outcomes from five years before and five years after that practice started. As it turned out, birth rates at the clinic didn't suffer because of the policy -- in fact, they improved over time.

Before the single-embryo policy, 51 percent of all women younger than 38 got pregnant and gave birth to a live baby, compared to 56 percent afterwards.

And the proportion of women with births of multiples dropped from 35 percent to less than 18 percent when the policy was in place.

Among 364 women who fit the criteria and had a single-embryo transfer from 2004 on, 65 percent gave birth to a live baby and just over three percent had twins or other multiples.

Kresowik said that the study can "serve as a model" to other fertility clinics that want to lower their twin rates.

"A policy like this is a great idea," said Judy Stern, a fertility specialist at the Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire, who wasn't involved in the new study.

The findings, she told Reuters Health, show that "it can work. You really can move toward single-embryo transfer and have it give you great success rates, and really low multiple rates."

Dr. Nicole Noyes, from the New York University Fertility Center, said that her team has also seen success using single-embryo transfer in ideal cases.

If doctors find good-quality embryos, she told Reuters Health, "and you have a good prognosis patient, putting a single embryo in does not lower pregnancy rates."

Another important finding, Kresowik said, was that the clinic didn't have any fewer women seeking treatment after the policy was implemented. Some women hope for twins during IVF, she said, seeing it as "an efficient way to reproduce" if they aren't fully aware of the risks involved -- but they didn't seem to flock away.

"If your practitioner's telling you, 'I think you should only have one embryo implanted,' that's a good thing," she concluded.

SOURCE: http://bit.ly/qjjdpu Fertility and Sterility, online September 29, 2011.