Many women having IVF could be treated with a single embryo without lowering their chances of having a child, according to research into a procedure that could play a pivotal role in reducing hazardous twin and triplet births.

British doctors have discovered that transferring one embryo at a time to the womb can slightly increase success rates, provided that patients are chosen carefully and a new culture technique is used.

The study, led by Yacoub Khalaf, of Guy’s Hospital, London, is the first in Britain to demonstrate that IVF can be as effective with one embryo as with two, and will increase pressure on fertility clinics to adopt similar methods to reduce multiple births.

While many childless couples see a multiple birth as an ideal way to complete their families in one go, such pregnancies are by far the biggest health hazard of fertility treatment. Twins and triplets are much more likely than singletons to be stillborn, to die in the first week of life, to be disabled or to be born prematurely.

The Human Fertilisation and Embryology Authority estimates that 126 IVF babies died in 2003 who would have survived had they been singleton births.

The authority is presently consulting on proposals to discipline clinics that produce twins or triplets in more than 10 per cent of pregnancies.

The problem is caused almost entirely by the common practice of transferring more than one embryo to the womb, which is performed in 90 per cent of British treatment cycles to maximise the chances that one will implant and develop.

A pilot programme at Guy’s, however, has shown that in women under 35 who produce at least four good-quality embryos, one embryo can be transferred without compromising success rates.

The chances of an ongoing pregnancy actually climbed slightly for the clinic’s patients under 35: from 35 per cent to 41 per cent. Their multiple pregnancy rate was almost halved, from 37 to 19 per cent, and almost all the twins were conceived by women who used two embryos. Almost 200 Guy’s patients have been treated with the technique, and there have been nearly 100 births or ongoing healthy pregnancies.

The high success rate with single embryos was made possible by growing embryos in culture for five days, rather than the usual three, before transfer to the womb. The extra time allows the embryos to become blastocysts – balls of about 100 cells – which are more likely to implant and easier for embryologists to assess for quality. Blastocyst transfer is not appropriate for every woman, because not every embryo will develop into a blastocyst. It is usually reserved for those who produce plenty of embryos, about 20 per cent of IVF patients. As these tend to have the best prognosis, and therefore the highest chance of conceiving twins, the new technique could have a dramatic effect on multiple birth rates.

Mr Khalaf, who presented the results to the HFEA’s annual conference last week, said his clinic’s experience shows that it is possible to tackle the multiple birth problem without reducing patients’ chances of starting a family.

“It is a myth that single embryo transfer has to lower the success rate,” he said. “If you select the right patients, and use blastocyst transfer, it can be just as good. We believe firmly that a twin pregnancy is not an ideal outcome. People think it is two for the price of one, but the risks are real and we see heartache time after time. I would encourage every clinic to look at our results.”

Mr Khalaf’s team is also offering single blastocyst transfer to women aged 35-39, and those who produce at least three good embryos, to try to reduce multiple births even more.

In numbers

1 in 80 Multiple births after natural conception

1 in 4 Multiple births after IVF

11,941 Number of babies born in multiple births in 1978

18,395 Number of babies born in multiple births in 2003

40% Proportion of IVF babies that are twins

90% Proportion of British IVF cycles performed with more than one embryo

50% Proportion of twins who ware born underweight

5 times Risk of death in first week of life for twins compared with singletons

35% Previous success rate at Guy’s for women under 35 with one or two embryos

41% Success rate for women under 35 with single blastocyst transfer

Source: HFEA, Guy’s Hospital

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