Frozen embryos are more likely to produce successful, complication-free IVF pregnancies than those that are fresh, research suggests.
Using stored embryos cuts the risk of bleeding in pregnancy, premature birth, and giving birth to an underweight baby by almost a third, a study has found.
The risk of a baby dying at around the time of birth is also reduced by about a fifth.
If the findings are confirmed it could have major implications for the public funding of In-Vitro Fertilisation treatment.
Currently the NHS regards embryo freezing as an extra service patients are expected to pay for themselves.
If freezing becomes a routine part of IVF treatment there may be pressure to change this rule.
Scientists made the discovery after analysing data from 11 international studies involving more than 37,000 IVF pregnancies.
In some cases, newly conceived fresh embryos were used. In others, embryos that had been frozen and stored for two to three months were implanted.
Standard practice is to choose the best embryos for fresh transfer, and only freeze those of good enough quality that are spare.
But the new results suggest it might be wise to freeze all embryos.
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Dr Abha Maheshwari, senior lecturer at the University of Aberdeen and Consultant in Reproductive Medicine with NHS Grampian, said: "We found pregnancies arising from the transfer of frozen thawed embryos seem to have better outcomes both for mums and babies when compared to those after fresh embryo transfer.
"If pregnancy rates are equal and outcome in pregnancies are better, our results question whether one should consider freezing all embryos and transfer them at a later date rather than transferring fresh embryos. This represents a major paradigm change in assisted reproduction, and one which could satisfy the twin demands of optimising safety and success."
Dr Maheshwari presented her findings at the British Science Festival, which opened today at the University of Aberdeen. The research also appears in the journal Fertility Sterility.
The scientists think there could be two reasons for the results. One is that only prime quality embryos are likely to survive the freezing process. The other, more popular, theory is that the womb lining is allowed time to settle down and recover from the rigours of IVF hormone treatment.
A pregnancy too soon after a women is given fertility-boosting drugs is known to increase the risk of hyperstimulation.
Dr Maheshwari added: "The existing data do have a number of limitations which need to be addressed in the context of further research before this strategy should be rolled out into routine clinical practice.
"The initial step must be to provide robust evidence to demonstrate that elective freezing of embryos can increase the chances of having a healthy baby, which would be best performed in the context of a large randomised controlled trial.
"In the meantime my advice to women undergoing IVF is that there is no reason, yet, to change the way they approach IVF.
However, there should be no concerns about freezing embryos and resulting pregnancies, if your clinic is offering the freezing of spare embryos."
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Allan Pacey, senior lecturer in reproduction and developmental medicine at the University of Sheffield, 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.
"I'd have to sound a cautious note about suggesting that all IVF embryos should be frozen, because the study doesn't support that. It only looked at successful frozen cycles. What it does do is give support to the idea that transferring one fresh embryo and freezing the others is okay.
"Freezing all embryos would have a catastrophic effect on pregnancy rates."
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