Call for debate on freezing IVF embryos
Fertility doctors have called for a debate around whether freezing embryos should become the main option for IVF treatment in the future.
An analysis of 13,000 IVF pregnancies suggested the freezing process might be better for the mother and the baby's health.
However, some fertility specialists argue there would be fewer pregnancies if freezing was more widely used.
The study's findings were presented at the British Science Festival.
Most of the time in IVF clinics in the UK, eggs are taken, fertilised and the resulting embryos implanted. This is thought of as using fresh embryos.
However, about one in five cycles of IVF in the UK uses frozen embryos - these were "spare" embryos kept from a previous IVF attempt.
There have been concerns that freezing may pose a health risk. However, the latest analysis, which is also published in the journal Fertility and Sterility, suggests that freezing may have better results.
It reported a lower chance of haemorrhage, premature birth and deaths in the first few weeks of life.'Debate now'
Lead researcher Dr Abha Maheshwari, from the University of Aberdeen, said: "Our results question whether one should consider freezing all embryos and transfer them at a later date rather than transferring fresh embryos."
She told the BBC more research was needed and that it was "a controversial topic".
"It is a debate we should be having now," she added. "It needs further exploration about what we do in the future."
Why frozen embryos might have better results is unknown and the researchers acknowledge the results are "counter-intuitive".
One theory is that stimulating the ovaries to release more eggs, as part of normal IVF, may affect the ability of the womb to accept an embryo. Freezing the embryo until later would allow it to be implanted in a more "natural" womb.Fewer births
However, data from the Human Fertilisation and Embryology Authority says that in 2010 frozen embryos were less likely to result in pregnancy. There was a 23% success rate for frozen and a 33% chance for fresh embryos.
Dr Maheshwari argues that new techniques in the past few years have greatly increased the success rate.
However Prof Alison Murdoch, the head of the Newcastle Fertility Centre at Life, Newcastle University, said: "It is of some concern that conclusions have been drawn, incorrectly, that we should routinely freeze all embryos and transfer them in a future menstrual cycle.
"There is ample evidence to show that this would result in fewer pregnancies even if the outcome for those pregnancies were better."
The director of IVF at Hammersmith Hospital in London, Stuart Lavery, said it would be "incorrect to conclude from these findings that we should stop performing fresh transfers and freeze all embryos".
However Mr Lavery said it provided reassurance that frozen embryos were as safe as fresh ones.
It was a view shared by Dr Allan Pacey, the chairman of the British Fertility Society and a researcher at the University of Sheffield.
He said: "I think this is interesting because some people are nervous about frozen embryos and there have been various headlines about this study or that which suggest that frozen embryos may be a worry.
"What's really useful is that it shows that from the point of view of the woman's health during labour, and some early measures of the baby's health, frozen embryos do all right and are arguably better."