Embryos can be lost through the freezing process
More evidence has emerged that babies born from frozen embryos are healthier than those that develop from fresh embryos, researchers say.
Three studies presented to a US fertility conference found frozen embryo babies were less likely to be premature and under weight.
Previous research has suggested this is down to only the strongest embryos surviving the freezing process.
Fertility experts said more work was needed on the issue.
A Finnish study, to be presented to the American Society for Reproductive Medicine meeting in San Francisco, found that babies born from fresh embryos were 35% more likely to be premature and 64% more likely to have low birthweight than those born from frozen.
A second study, from the University of Pennsylvania in Philadelphia, found that babies born from fresh embryos were 51% more likely to have low birthweight and were 15% more likely to die around the time of birth than those born from frozen embryos.
And research by the Royal Women's Hospital in Melbourne, Australia, found 11% of babies born from fresh embryos had low birthweight compared with 6.5% of babies born from frozen.
A total of 12.3% of babies born from fresh embryos were premature, compared with 9.4% of those born from frozen, while 1.9% also suffered death compared with 1.2% from frozen.
It is thought the results were related to the quality of the placenta, the digestive and respiratory system for the foetus.
The Australian researchers said the findings suggested women may prefer to use frozen embryos in the future.
But Dr Allan Pacey, a fertility expert from the University of Sheffield and secretary of the British Fertility Society, warned it was not so clear cut.
"Frozen embryo transfers are not as successful as fresh ones in terms of getting a pregnancy.
"So it may be that we have to balance the health of children against chances of success."
But he added: "It is intriguing research that goes against what we would normally assume. It now needs to be looked at again."