Women undergoing IVF treatment should have just one embryo implanted instead of two, researchers have said.
By Caroline Ryan
BBC News Online health staff in Berlin
Scandinavian researchers told the European Fertility Conference in Berlin single embryos give women virtually the same chance of becoming pregnant.
They said it meant woman and babies would avoid the risks associated with multiple births.
But the UK's fertility authority said more evidence was needed before it could recommend use of just one embryo.
Over 660 patients were studied in the Scandinavian research, which took place in centres in Sweden, Norway, Denmark.
All the women were under 36 years of age, and undergoing a first or second IVF cycle, and had at least two good quality embryos available for transfer or freezing.
They either had two embryos implanted, or one embryo implanted and one frozen for use in a second cycle if there was no live birth from the first.
The pregnancy rate among those given single embryos was 39.7%, compared to 43.5% in the double embryo group.
Ninety one of the single-embryo women became pregnant after their first cycle of treatment. Another 29 became pregnant after the second.
In the double embryo group, 142 women became pregnant.
Dr Ann Thurin of Sahlgrenska University Hospital, Goteborg, Sweden, who led the study, told BBC News Online: "You get the same live births [from single embryo transfers], but you don't have the multiple births, which is the main problem."
Multiple births tend to lead to smaller babies, and a higher risk of health problems for both the babies and their mother.
She said she would recommend all European countries introduced the single embryo transfer as standard for women in this group - under 36, in their first or second cycles who have good quality embryos.
The Human Fertilisation and Embryology Authority, which regulates fertility treatment in the UK, ruled earlier this year that a maximum of two embryos should be implanted in each cycle of treatment.
A spokeswoman said: "One of the reasons the HFEA is at this conference is to look at the evidence that comes from other countries."
But she said more evidence was needed before the HFEA recommended a move to single embryo transfer.
"We have stated that we would like to move to single embryo transfer, but we don't want to do that if it's going to disadvantage anyone.
"If there was an age limit, we would have to get it right."