British IVF clinics have used frozen embryos for years

Scientists in the United States looked at 19 women who had this treatment and found that six went on to lose their baby because of the condition.

This rate of over 30% is much higher than that for women who received IVF treatment with fresh embryos.

British experts said because of the small numbers involved the findings should be interpreted cautiously.

Dr David Keefe and colleagues at Brown University and Women and Infants Hospital in Providence, Rhode Island, studied the medical records of women who had IVF treatment at the hospital between January 1998 and March 2002.

Of the 490 pregnancies from fresh IVF cycles only nine or 1.8% were ectopic. This compares to the 31.6% rate for women who were treated with frozen embryos.

Outside the womb

An ectopic pregnancy occurs when the embryo implants itself outside instead of inside the womb. This can take place in the fallopian tubes, ovary, abdomen or cervix.

I would be very cautious about drawing any conclusions from these findings Richard Kennedy, British Fertility Society

The condition is potentially life-threatening. The embryo can rupture the fallopian tube, leading to massive internal bleeding - and possibly death - for the mother, and loss of the baby.

The condition affects around one in 100 pregnant women.

IVF clinics in Britain have been using frozen embryos for over 15 years.

Richard Kennedy of the British Fertility Society said there was no reason to believe frozen embryos carried a higher risk of ectopic pregnancy.

"We have been using frozen embryos for over 15 years and have carried out thousands of frozen embryo cycles.

"Nobody has previously reported an increased risk from this type of technique," he told BBC News Online.

"There is no anatomical or biological reason why using a frozen embryo would increase the risks of an ectopic pregnancy.

"I would be very cautious about drawing any conclusions from these findings, not least because of the small numbers involved."

The Human Fertilisation and Embryology Authority, which regulates IVF treatment in the UK, said it would examine the findings.

"We look at any study that impacts on IVF in this way," a spokeswoman said.