In vitro method found to reduce multiple births

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Transferring one embryo during in vitro fertilization reduces the risk of multiple births and also leads to higher delivery rates when the lone embryo is received at a slightly later stage of development, according to a new study.

The study by Belgian researchers, published in last week's issue of the New England Journal of Medicine, involved 351 women younger than 36 who were undergoing a first or second trial of in vitro technology or another technology designed to help them overcome infertility.

The findings could have implications in the United States, where it is routine to transfer two or more embryos during in vitro fertilization, according to an editorial accompanying the report.

Laura A. Schieve of the National Center for Birth Defects and Developmental Disabilities, who wrote the editorial, noted that about 35 percent of U.S. births in 2002 resulting from assisted reproduction technology "included twin or higher-order multiple gestations."

In contrast, Ms. Schieve said, "the incidence of multiple births after natural conception is less than 2 percent."

But in the Belgian study, led by Dr. Evangelos G. Papanikolaou, a reproduction specialist at Vrije University in Brussels, the multiple-birth rate was equivalent: about 2 percent. Of the 94 deliveries that resulted among study participants, just two involved twins, both of them identical pairs.

All women in the Belgian study received only one embryo. Half got embryos that had been cultured for three days, a stage described as "cleavage-stage," while the other half got embryos cultured for five days, which are called "blastocyst-stage."

The purpose of the research was to compare the rates of continued pregnancy and live delivery in women who received the older versus the younger embryos. Transferring cleavage-stage embryos is far more common than implanting blastocyst-stage embryos.

"As compared with the cleavage-stage group, the blastocyst-stage group had a significantly higher rate of both ongoing pregnancy (33.1 percent versus 21.6 percent) ... and delivery (32.0 percent versus 21.6 percent)," the authors wrote.

The experiment was stopped early, given the much better results with blastocyst-stage embryos. The authors noted that the two sets of identical twins who were born came from cleavage-stage embryos.

"Clearly, the most effective strategy for a reduction in the number of multiple births is to transfer a single embryo during a procedure," said Ms. Schieve, whose office is part of the Centers for Disease Control and Prevention.

But she acknowledged this option "may be unacceptable" for many infertile women, who seem to think their odds of live-birth delivery would be lower with the transfer of a single embryo, rather than two or more. But "multiple gestation still remains a substantial risk," Ms. Schieve observed.

She said that culturing embryos for five days, rather than three, "may allow for more accurate assessment of embryo quality and thus for enhanced selection of high-quality embryos."

But citing the U.S. trend of women postponing motherhood, Ms. Schieve said the value of a European study that restricted participation to women younger than 36 is limited.

"Less than half of women undergoing treatment with assisted reproductive technology in the United States would meet the age criterion alone," she said.