WEDNESDAY, Jan. 11 (HealthDay News) -- The ideal number of embryos to implant during in-vitro fertilization (IVF) therapy can be one or two, but should never be more than three, according to a new study that examined this controversial issue.

British researchers analyzed data from about 124,000 IVF cycles that resulted in 33,500 live births. The live birth rate was higher among women of all ages who had two embryos implanted than among those who received one embryo.

But among women younger than 40, transferring three embryos was associated with a lower birth rate than transferring two embryos. The success rate for women older than 40 who received three embryos was the same as for those who received two embryos.

Compared with transferring one embryo, transferring two or three embryos was associated with a higher risk of all adverse perinatal outcomes. The risk for severe preterm birth was particularly high among women who received three embryos, according to the study.

Overall, the live birth rate was lower in older women than in younger women, no matter how many embryos were transferred.

The study appeared online Jan. 11 in The Lancet.

"In older and younger women, the transfer of two embryos was associated with greatest live birth rates...," Debbie Lawlor, of the University of Bristol, and colleagues said in a journal news release. "A clear implication of our study is that transfer of three embryos should no longer be supported in women of any age."

"In both age groups, transfer of three embryos did not increase the livebirth rate over that seen after transfer of two embryos, but was associated with an increased risk of adverse perinatal outcomes."

In younger woman with only two or a small number of embryos successfully fertilized, transferring two embryos, rather than just one, as is legislated in some countries currently, may be justified, the researchers noted.

"In view of these findings and previous findings, we conclude that no more than two embryos should be transferred into women of any age, but greater freedom should be given to clinicians and patients to decide whether to transfer one or two embryos, according to prognostic indicators, such as maternal age," they added.

Couples need to be aware that delaying childbirth may make it difficult for women to conceive and harder for in-vitro fertilization to be successful, the researchers added. For doctors seeing a new couple struggling with infertility, starting IVF quickly is key when the woman is in her late 30s compared to her late 20s, the researchers suggested.

More information

The MedlinePlus Medical Encyclopedia has more about IVF.