Three new independent studies have provided further evidence that embryos stored using slow-freezing techniques may be better than fresh for IVF. The studies were presented at the American Society for Reproductive Medicine conference in San Francisco, US, last week. The studies indicate that using frozen embryos rather than fresh embryos reduces the risk of stillbirth and premature delivery.

It is unclear why this is the case; there are several theories. Some experts have suggested that when fresh embryos are used women may still be suffering from the effects of the powerful drugs that are used to stimulate the ovaries, temporarily disrupting any IVF attempt shortly afterwards. Dr Allan Pacey, from the University of Sheffield and secretary of the British Fertility Society (BFS), said: 'These findings are really quite interesting. It kind of defies logic to a certain extent, because the stimulation drugs and anaesthetics that are used in egg collection have worn off by the time fresh embryo transfers are done. It seems to be an issue with the formation of the placenta, but how it has an effect isn't known.' Another theory is that only the 'best' embryos surviving the freezing and thawing processes, which can damage the embryo either as the result of ice crystals forming or of an increase in the concentration of solutes as progressively more ice is formed.

The three large, independent studies took place in Finland, Australia and the US. The Finnish study, found that babies born from fresh embryos were 35 per cent more likely to be premature and 64 per cent more likely to have a low birth weight when compared to those born from frozen embryos. The research that took place in Melbourne, Australia, showed that 11 per cent of babies born from fresh embryos had a low birth weight, compared to 6.5 per cent of those born from frozen embryos. They also found that 12.3 per cent of babies born from fresh embryos were premature, compared with 9.4 per cent of those born from frozen embryos. Also, 1.9 per cent of babies from fresh embryos died a few days after birth, compared to 1.2 per cent from frozen embryos. Similar findings were reported in June this year from a Danish study.

Typical IVF treatment involves stimulating a woman's ovaries with hormones to produce eggs which are then collected and fertilised in the laboratory, with one or two embryos being transplanted into the womb two days later