Implanting a single embryo is the cheapest and most effective way for women to have a healthy baby through fertility treatment, Finnish researchers said on Wednesday.

The findings from the long-running study counter fears that relying on just one embryo could drive up treatment costs and reduce a woman's chances of giving birth to a full-term baby, the researchers said in the journal Human Reproduction.

"At a time when there is an intense debate in many countries about how to reduce multiple pregnancy rates and provide affordable fertility treatment, policy makers should be made aware of our results," said Hannu Martikainen of the University of Oulu in Finland, who led the study.

"These data should also encourage clinics to evaluate their embryo transfer policy and adopt single embryo transfer as their everyday practice for women younger than 40."

Multiple pregnancies are the biggest risk for women during fertility treatment and significantly increase the likelihood of miscarriage, premature birth and long-term health problems for the child. They can also endanger a mother's health.

More than 3.5 million babies have been born worldwide using assisted reproductive technology since July 25, 1978, when British doctors delivered the world's first test-tube baby, Louise Brown.

The technique involves surgically removing eggs from a woman's ovaries and combining them with sperm in the lab. Doctors then pick the best embryos — typically one or two — and implant them in the uterus.

The debate over multiple embryos was reignited this year after a California woman with six children received fertility treatment and gave birth to eight newborns.

The Finnish team compared the outcomes for 1,510 women seeking fertility treatment at a single clinic between 1995 and 1999 when double embryo transfer was more common and from 2000 to 2004 when single embryo transfers were more widespread.

They found that more babies were born at term for women who received a single embryo, with birth rates per woman of nearly 42 percent compared to about 37 percent for those who received multiple embryos.

Single embryo transfers are also cheaper, especially when it comes to costs related to treating complications caused by multiple births, the researchers said.

"This is the biggest evidence yet that single embryo transfer works and it works well," Zdravka Veleva, who worked on the study, said in a telephone interview.