But a growing number are having multiple births.

HFEA deputy chairman Jane Denton explains how the guide can help

The finding comes in the Human Fertilisation and Embryology Authority's annual report and interim patients' guide, which publish the number of live births per in-vitro fertilisation (IVF) treatment cycle for the UK's 106 specialist fertility treatment and storage units over the period 1996-1997.

A live birth is counted as the successful delivery of one or more babies, and the live birth rate is the number of live births per 100 treatment cycles.

The report shows that the overall live birth rate was 16.7% in 1996-1997, up from 15.2% in 1995-1996.

One of the reasons quoted for the rise in success is the increased use of the ICSI - intra-cytoplasmic sperm injection - treatments.

This involves a sperm being injected directly into an unfertilised egg and is fairly controversial.

Overall, 7,292 babies were born as a result of IVF treatment in the period covered compared to 6,130 the previous year.

However, the HFEA warns that there are still "too many" multiple births occurring as a result of fertility treatment.

It reports that last year almost half - 47% - of all babies born as a result of in-vitro fertilisation came from a multiple birth. The authority estimates that at least half of all sets of triplets born in the UK were born as a result of IVF treatment.

High chance

Multiple births often happen in the course of fertility treatment because doctors normally return two or three embryos to the womb after the eggs have been fertilised.

The number of babies born as a result of fertility treatment increased last year

The HFEA figures published on Tuesday show that there is little difference in the success rate between cases where two embryos are returned and when three are put back - the live birth rate is 20% when two embryos go back and 22.5% when it is three.

However, the multiple birth rate has increased from 22.4% in 1995-1996 to 32.2% in the last year.

Ruth Deech, chairman of the HFEA, said: "Multiple births can be the source of much stress and anxiety for parents.

"There is a greater risk of complications or even miscarriage, as well as long-term disability, and they can cause considerable emotional and financial pressure.

"People undergoing IVF treatment should be aware that as the technique becomes more successful, so the risk of a multiple birth increases."

She said that the HFEA, which regulates and licenses fertility clinics under the 1990 Human Fertilisation and Embryology Act, would continue to look into the matter.

Baby boom

The HFEA report found that there had been a significant rise in the use of the ICSI technique over the last year, and the number of babies born had risen five-fold.

ICSI is particularly useful for men with low sperm counts as it takes sperm that would not otherwise be able to make it to an egg and places them directly inside it.

Mrs Deech said: "This revolutionary technique has enabled many men who previously would never have been able to have their own children to do so.

"A highly delicate procedure, its initial success rate of around 4% per treatment cycle has risen dramatically to 21.6%."

However, some doctors have called for people receiving the treatment to be given counselling beforehand, as some research has shown there could be a danger of genetic defects in a proportion of children born through ICSI.