Women who have fertility treatment with frozen embryos may have a better chance of success than those who use "fresh" ones, a study has found.
The findings represent a "major paradigm change" in assisted reproduction and could lead to a change in the current policy of preferring fresh embryos to frozen ones, scientists said.
A systematic review of more than 37,000 IVF pregnancies showed that there was a reduced chance of mothers bleeding and of babies being born prematurely with low birth weight when the embryos were stored and frozen compared with treatment involving freshly-transplanted ones.
The review of 11 international studies also found that the death rate immediately before and after birth was lower in babies who were born as a result of transferring frozen-thawed embryos into the womb compared with the transfer of fresh IVF embryos.
Scientists said the results should reassure mothers who intend to have IVF treatment with frozen embryos, but opinions are divided about whether the findings should lead to a change in the existing policy.
"If pregnancy rates are equal and outcome in pregnancies are better, our results question whether one should consider freezing all embryos and transferring them at a later date, rather than transferring fresh embryos," said Abha Maheshwari, a lecturer at the University of Aberdeen who led the study published in Fertility and Sterility.
"This represents a major paradigm change in assisted reproduction, and one which could satisfy the twin demands of optimising safety and success. Traditionally it has been thought that fresh is always better and used as a first choice," Dr Maheshwari said.
"Initially there were concerns about the safety of freezing techniques, and it was felt the quality of the embryo could deteriorate and impact on the health of the child. However, data to date has been reassuring," he told the British Science Festival in Aberdeen.
Current practice for most IVF clinics is that the best, fresh embryos are used first and only spare, good-quality embryos are frozen for transfer at a later stage. Freezing embryos has become more popular since single embryos have been transferred, rather than the riskier transfer of two or more embryos.
But Professor Alison Murdoch, head of the Newcastle Fertility Centre, said the findings of the review did not justify a change in existing policy.
"The results of individual studies that are considered in this meta-analysis are already being discussed in clinics and 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," she said.
Peter Braude, a fertility specialist at King's College London, said the results of the study were counter-intuitive since "second-best" embryos are usually selected for freezing, with the best being transferred immediately.