Women who initially receive just one embryo during in vitro fertilization are as likely to produce a child as women implanted with two, Swedish researchers reported on Wednesday.
The research, which follows up on a 2004 study, comes amid widespread concern over health risks to mothers and babies associated with multiple births that often occur when more than one embryo is implanted using test tube baby technology.
Dr. Ann Thurin-Kjellberg of Gothenburg University and her colleagues report in the New England Journal of Medicine that they initially gave 661 women either one or two embryos.
If the women didn't become pregnant, they received embryos that had been frozen and thawed. One or two frozen embryos were implanted for each attempt, and up to four such attempts were made.
More than half, 53 percent, of the women implanted with one embryo on the initial try gave birth, compared to 57 percent who received two embryos on the first attempt, an insignificant difference, the researchers said.
But there was a big difference in the odds of having twins or premature babies.
The multiple birth rate was nearly 28 percent and the risk of a premature baby more than doubled among the women who initially received two embryos, compared to a multiple birth rate of 2.3 percent for women who started off in the single-embryo group.
"This research adds further evidence confirming the value of elective single embryo transfer in assisted reproductive technologies," said Dr. Richard Kennedy, a spokesman for the International Federation of Fertility Societies.