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Single Egg Implants Reduce Chances of Multiple Births

Modified in vitro fertilization strategy maintains same rate of live births, study finds

WEDNESDAY, Dec. 1, 2004 (HealthDayNews) -- A modified strategy of in vitro fertilization (IVF) cut the rate of multiple births in younger women while keeping the rate of live births almost the same.

The strategy, done in women under the age of 36, involved transferring only one fresh embryo -- instead of two -- and then later, if the first embryo didn't "take," transferring one frozen-and-thawed embryo, Swedish researchers report.

"The benefit is the dramatic decrease in multiple birth rate while no substantial reduction in the overall live birth rate," said Dr. Christina Bergh, lead author of the study appearing in the Dec. 2 issue of the New England Journal of Medicine. Bergh is a professor of obstetrics and gynecology at Sahlgrenska University Hospital in Goteborg, Sweden.

An estimated 1.2 million children have been born as a result of IVF worldwide, and about 1 percent of newborns in the United States are conceived with assisted reproductive technology, according to an accompanying editorial in the journal.

IVF greatly increases the odds of having twins, triplets or even more babies. And multiple births are a concern because they heighten the risk of premature birth, low or very low birth weight, and even infant death. Neonatal mortality is four times higher among twins than single babies, and twins are also at increased risk for long-term disabilities, including cerebral palsy, according to the editorial.

The practice of implanting multiple embryos dates from the introduction of IVF, about 25 years ago, when successful pregnancy rates were extremely low.

"IVF has come a long way since the early days when you would put in four embryos and have a 15 percent pregnancy rate," said Dr. Owen K. Davis, author of the editorial, and associate director of IVF at Weill Medical College of Cornell University in New York City.

"Now that each embryo has a better chance of implanting than they used to, we have the luxury of worrying about multiple births," added Davis, immediate past president of the Society for Assisted Reproductive Technology.

In 1993, Swedish physicians started to reduce the number of embryos transferred into a woman from three to two. As a result, hardly any triplets are born there, while the pregnancy rate remains at 35 percent and the live birth rate at 25 percent, the researchers said.

In the United States, guidelines call for transferring no more than two embryos and considering only one when possible.

For the current study, conducted in Sweden, women under the age of 36 with at least two good-quality embryos were randomly assigned to one of two groups. Women in the first group underwent a transfer of two fresh embryos at one time. Those in the second group had a single fresh embryo first and, if there was no live birth, a subsequent transfer of a single frozen-and-thawed embryo.

In the two-fresh-embryo group, 142 of 331 women (42.9 percent) had pregnancies resulting in at least one live birth. In the second group, 128 of 330 women (38.8 percent) had pregnancies resulting in at least one live birth (only 27 percent became pregnant after one transfer while the rest had to have a second transfer).

However, the rates of multiple births were 33.1 percent for the two-embryo group and 0.8 percent for the single-embryo group, the study said.

"This is the first study with enough power to test the hypothesis that one embryo transferred at one to two occasions compared to a conventional two-embryo transfer at one occasion would give similar live birth rates," Bergh said. "The study emphasizes the importance of a well-functioning freezing program and I think clinics should try to improve their results after freezing."

It's important to note that the women participating in the study were likely to become pregnant from IVF, Davis emphasized. They were young (average age was about 31), this was only their first or second attempt at IVF, and they each had at least two viable embryos.

"In my clinic, the average age is considerably older and a lot of patients have failed multiple times," Davis said.

Which raises the question of whether these results, obtained in the relatively homogenous population of Sweden, are translatable to the United States. "In the U.S., unlike Europe, patients go to IVF as the last resort so they often have had other failed [fertility] treatments, whereas in Europe, there is a tendency to go straight to IVF," Davis said.

Also, only 50 percent to 75 percent of embryos survive thawing. "In this study, 17 percent of people that failed a single transfer" did not have the option of a second embryo, Davis said. "The study is basically an eye opener, making some very good points, but you can't necessarily say you're going to do this in everyone."

More information

Visit the American Society for Reproductive Medicine for more on in vitro fertilization.

SOURCES: Christina Bergh, M.D., Ph.D., professor, obstetrics and gynecology, Sahlgrenska University Hospital, Goteborg, Sweden; Owen K. Davis, M.D., associate director, IVF, Weill Medical College of Cornell University, New York City, and immediate past president, Society for Assisted Reproductive Technology; Dec. 2, 2004, New England Journal of Medicine
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