Two embryos are better than three.

Although in vitro fertilization 0VF) helps thousands of infertile couples to have children, it also yields multiple births roughly a fourth of the time. In the procedure, doctors typically take eggs from a woman's ovaries, fertilize them with sperm in a laboratory, then place them in the woman's uterus. IVF results in pregnancy in about one in five tries.

Multiple births, which occur in less than 2 percent of unassisted pregnancies, arise more often from IVF because doctors transfer several fertilized eggs, or embryos, to improve the odds. The United Kingdom restricts the number of embryos transferred to three at a time; the United States imposes no limits.

Seeking a way to limit multiple births, scientists in Scotland reviewed data from 25,240 women who had undergone IVF. The researchers report in the Aug. 27 New England Journal of Medicine that when more than four eggs were fertilized but only the two most robust-looking embryos were chosen, pregnancy resulted at the same rate as when women received three or more embryos. Moreover, women receiving two embryos instead of three had many fewer multiple births.

Fertilizing a greater number of embryos enables the doctors to have more selectivity, says coauthor Allan Templeton, an obstetric gynecologist at Aberdeen University. The best embryos, he says, "look nice and round and are dividing at a good rate."

"Clearly, there is pressure from patients to maximize the chances" of a successful pregnancy by using more embryos, Templeton says. "In the U.K., there's an increasing perception that in most circumstances putting back [into the woman] lots of embryos doesn't achieve very much."