IVF produces healthier children when embryos are frozen and thawed before being transferred to the womb, according to new research.

Babies born from frozen IVF embryos are less likely to be born prematurely or underweight than are those conceived during fresh treatment cycles, three independent teams of scientists have found. One of the studies also recorded lower rates of stillbirth and early death among frozen-embryo babies.

The results, from researchers based in the United States, Australia and Finland, suggest that far from being riskier than conventional IVF, as is generally thought, cycles using frozen embryos may actually be safer.

The research is particularly significant because separate teams from three countries have reached the same conclusion, each using large samples including thousands of IVF cycles. A fourth team, from Denmark, also reported similar findings earlier in the year.

Scientists said the work would be important because women are increasingly encouraged to use just one fresh embryo during IVF, to guard against damaging twin and triplet births, and to freeze any that are left over for later use. It will dispel doubts about whether this policy trades the risks of multiple births for hazards associated with frozen embryos.

Allan Pacey, secretary of the British Fertility Society, said: “This all works to the advantage of single embryo transfer. It suggests it might be better from the point of view of a healthy child to go for frozen embryos, which is an integral part of the strategy. People shouldn’t be scared of frozen embryos.”

The studies, presented at the American Society for Reproductive Medicine conference in San Francisco, do not explain why babies born from frozen embryos seem to do better, but there are several hypotheses.

When fresh embryos are used in treatment, women may still be under the influence of powerful hormonal drugs used to stimulate their ovaries into producing extra eggs, or the anaesthetics used during egg collection. Either of these may interfere with the endometrial lining of the womb, and thus with the formation of the placenta.

Another possibility is that as some frozen embryos generally fail to survive thawing, this effectively “weeds out” those that are of poor quality. A third possibility is that women who produce lots of good quality embryos are more likely to have frozen cycles, and also have a lower risk of complicated pregnancies.

“These findings are really quite interesting,” Dr Pacey said. “It kind of defies logic to a certain extent. It seems to be an issue with the formation of the placenta, but how it has an effect isn’t known.

“This may be a bit of biology that has escaped most people. The assumption has always been that if the endometrium [the womb lining] is thick, it’s functional. In an IVF cycle, the endometrium looks normal on ultrasound, but perhaps there is something subtle going on.”

Despite their apparent health benefits, however, frozen IVF cycles are unlikely to supplant fresh ones as the routine option because they have a lower success rate. In Britain, 24.4 per cent of fresh cycles lead to a live birth, compared to 17.4 per cent of frozen cycles. There were 33,916 fresh cycles performed in 2006, and 7,911 frozen cycles.

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