More than 68,000 babies have been born in Britain as a result of in-vitro fertilisation. The worldwide figure is nearer 1 million.

More than 68,000 babies have been born in Britain as a result of in-vitro fertilisation. The worldwide figure is nearer 1 million.

A number of studies have suggested these children suffer no greater risk of congenital or other abnormalities than naturally conceived children. But recent research has pointed to longer-term problems that were possibly overlooked.

One of the first studies, by the Medical Research Council, catalogued all IVF births between 1978 and 1987 for the first year of life and found that 2.2 per cent of the 1,581 babies showed congenital malformation. This was on a par with naturally-conceived babies.

However, doubts began to emerge about the wider safety of IVF. The single biggest risk appeared to result from multiple pregnancies which often occur following the implantation of more than one embryo into the womb. Doctors do this in order to increase the chances of a successful pregnancy and many countries, including Britain, now limit the number of implanted embryos to two.

A second area of concern is the growing use of frozen embryos and eggs and a technique of directly injecting a single sperm into an unfertilised egg, called intra-cytoplasmic sperm injection (ICSI). A study by a team in Belgium who pioneered ICSI has found that these children suffer no significantly increased health risks compared to other IVF children but some fertility specialists, including Lord Winston, believe this study to be incomplete. Sceptics suggest that any technique which involves directly injecting potentially defective sperm into an egg will increase the risk of inherited defects being passed on to children.

A study published this year by a team of Australian scientists found 75 out of 837 infants – 9 per cent – born after IVF treatment and 26 out of 301 born after ICSI – 8.6 per cent – had a major birth defect. This is roughly twice the rate of birth defects in a control group of 4,000 naturally conceived embryos from a similar population.

"Reports of a treatment that leaves almost 10 per cent of children handicapped in some way must not be dismissed as a mere statistical quirk," says Lord Winston and his colleague Kate Hardy in an article published in the current issue of Nature Cell Biology. "What's more, this disquieting trend was still apparent when these worrying figures were corrected for multiple births, maternal age and parity [number of children] – even IVF singletons were at considerable risk."

Many studies have failed to disentangle the role played by ICSI and cryopreservation (freezing) of embryos from the other risk factors of IVF. They have also not followed up the longer-term health of IVF children, many of whom are approaching adolescence.

"We must now be more vigorous in our surveillance of all these technologies," say Lord Winston and Hardy. "Patient desperation, medical hubris and commercial pressures should not be allowed to be the key determining features in this generation of humans ... We cannot ignore the clouds lowering over these valuable therapies."