The American Society for Reproductive Medicine is trying to make it the norm in the U.S., too. Its guidelines, updated earlier this year, say that for women with reasonable medical odds of success, those under 35 should be offered single embryo transfer and no more than two at a time. The number rises with age, to two or three embryos for women up to 40, since older women have more trouble conceiving.
To add heft to the advice, the guidelines say women should be counseled on the risks of multiple births and embryo transfers and that this discussion should be noted in their medical records.
"In 2014, our goal is really to minimize twins," said Dr. Alan Copperman, medical director of Reproductive Medicine Associates of New York, a Manhattan fertility clinic. "This year I'm talking about two versus one. Several years ago I was talking about three versus two" embryos.
The one-at-a-time idea is catching on. Only 4 percent of women under 35 used single embryos in 2007 but nearly 12 percent did in 2011. It's less common among older women, who account for fewer IVF pregnancies, but it is gaining among them, too.
"Patients don't really want multiples. What they want is high delivery rates," said Dr. Richard T. Scott Jr., scientific director for Reproductive Medicine Associates of New Jersey, which has seven clinics in that state.
Better ways to screen embryos can make success rates for single embryos nearly as good as when two or more are used, he contends. The new techniques include maturing the embryos a few days longer. That improves viability and allows cells to be sampled for chromosome screening. Embryos can be frozen to allow test results to come back and more precisely time the transfer to the womb.
Taking these steps with single embryos results in fewer miscarriages and tubal pregnancies, healthier babies with fewer genetic defects and lower hospital bills from birth complications, many fertility specialists say.