Most IVF treatments involve putting more than one embryo back into the woman, as historically this is believed to increase the chances of success.

However, it also means that there is a far greater chance of twins than normal.

And the latest research suggests that in many women, putting back just one embryo will yield the same chance of a successful pregnancy as two or more.

Although it [multiple pregnancy]has decreased in most of the European countries it remains at an unsatisfactory level

Professor Emile Papiernik, Hopital Port Royal, Paris

A study presented at a European fertility conference on Monday found that babies who are part of multiple pregnancies are more likely to die before birth, and have a significantly increased risk of ill health.

This is because they tend to be born premature, increasing the chance of death or poor health.

The findings will increase worries about the glut of multiple pregnancies from fertility treatment.

In the UK, the Human Fertilisation and Embryology Authority introduced new rules last year limiting the number of embryos that can be used in fertility treatment to two in all but exceptional circumstances.

However, there is debate whether this number is too high for many women, particularly younger patients.

French connection

French fertility experts carried out an analysis of nearly 25,000 pregnancies of women who had undergone assisted reproduction treatment between 1986 and 1998.

The pregnancies resulted in the births of 32,389 babies. Of these, 18,235 were singletons, 11,905 twins and 1,772 triplets.

Nearly nine out ten of the triplets, and over four out of ten of the twins were born prematurely, compared with just over 8% of single babies.

More than 5% of babies were born before 33 weeks of pregnancy with over 28% of these very premature babies being triplets, nearly 8% being twins and fewer than 2% being singletons.

Over 57% of the babies who were small for their gestational age were triplets, nearly 43% were twins and just over 17% were singletons.

Deaths in the period around birth were five times as frequent for triplets as for single babies (a rate of nearly 40 per 1,000 compared with under 8 per 1,000 for single babies). For twins it was just over 20 per 1,000.

Important factor

Lead researcher Professor Emile Papiernik, from Hopital Port Royal in Paris, said: "In total, twins and triplets represented 43% of the newborns, but 81% of the total preterm births.

"Multiple pregnancy is a very important risk factor for the babies' health.

"Although it has decreased in most of the European countries it remains at an unsatisfactory level."

His comments were echoed by Dr Karl Nygren, associate professor of obstetrics and gynaecology at Sofiahemmet Hospital in Stockholm, who told a news briefing at the conference that doctors could easily solve the problem.

Dr Nygren said the natural rate of multiple births was around 1%, but was between 20% and 40% for women who used assisted reproductive techniques.

He said this was because doctors tended to use more than one embryo at a time to increase the chance that each individual treatment led to a successful pregnancy.

This was partly due to patients wanting to maximise their chances of a quick pregnancy to keep costs down, and partly due to competition between doctors, keen to gain a reputation for good results.

To maintain efficacy we need to make a better selection of viable embryos and to identify patients at highest risk for multiple pregnancy

Karl Nygren, Sofiahemmet Hospital, Stockholm

However, new research by a Dutch team of fertility experts has produced preliminary evidence that transferring a single embryo to a woman's uterus seems as good a strategy for achieving a successful pregnancy as transferring two embryos at once.

Dr Nygren said the best way forward was to use just one embryo per treatment, but to select it more carefully so that it had a greater chance of producing results.

He said: "Doctors and patients are beginning to realise that the incidence of multiple pregnancy must be reduced.

"To maintain efficacy we need to make a better selection of viable embryos and to identify patients at highest risk for multiple pregnancy."

UK response

Ann Furedi, a spokeswoman for the HFEA, said the authority was aware of the Dutch research, and was keeping a close eye on developments.

However, she said: "Doctors are really trying to minimise the risk of multiple pregnancies, but at the same time they face pressure from patients who really want to maximise their chances of success through IVF."

In 1999 there were 23,254 cycles of IVF carried out in the UK. The success rate was 16.9% per cycle.

Just 3.2% of pregnancies involved the transfer of a single embryo.