We haven't designed anything

Child's father

The child, who will become the first in the UK to have been created in such a way, will be born by the end of the year.

The woman's four-year-old son is recovering from leukaemia.

However, there is a one in four chance of a relapse.

If that happens the child will need a bone marrow transplant.

A brother or sister created to have an immune system which is a perfect match would give a transplant operation the best possible odds of success.

It is hoped to harvest cells from the unborn baby's umbilical cord and freeze them to be used as a possible lifeline if the woman's son if he suffers a relapse of the cancer. The immature cells have the potential to develop into healthy bone marrow.

Discarded embryos

Zain Hashmi is seriously ill

However, the technology is highly controversial because it involves the screening of embryos - some of which will be discarded if they do not contain the right genetic code.

Speaking on condition of anonymity the couple dismissed accusations that they were creating a 'designer baby'.

The woman's husband is quoted in The Guardian as saying: "What have we designed? We haven't designed anything.

"All of these ethical dilemmas are very easy to conjure up when you haven't been put on the spot yourself."

His wife said: "At least we can look each other in the eye and say there was something out there that could have been done and we did it."

UK IVF specialist Mohammed Taranissi backed the couple's comments.

He said: "We are not creating designer babies. We are not trying to choose eye colour or hair colour.

"We are trying to prevent an illness."

A spokesman for Dr John Wagner at the University of Minnesota, who helped the couple get treatment, said she couldn't comment specifically on the case.

But she confirmed Dr Wagner was a "world leader" in the technique.

She said: "He takes embryos grown in the laboratory at the eight-stage cell and takes a cell biopsy, screening them for genetic abnormalities.

"In a case like this he would only implant a healthy embryo to ensure it will be a good, healthy bone marrow match."

A spokesman for the UK-based Human Fertilisation Embryology Authority said the case raised some difficult ethical questions.

"Is the child being born for its own sake or for someone else?

"We do not have international legislation on this. If people want to go outside, they can't be offered the same protection they have here."

Second case

Molly Nash was saved by her brother's bone marrow

Earlier this month Leeds couple Raj and Shahana Hashmi announced that they intended to seek permission to undergo a similar procedure in the UK to create a sibling to help their son.

Two-and-a-half year-old Zain has the potentially-fatal genetic disorder thalassaemia.

His parents want to create a brother or sister who may be able to donate cells for a life-saving transplant.

The Park Hospital's Centre for Assisted Reproduction in Nottinghamshire has agreed to help the couple.

But the Human Fertilisation and Embryology Authority (HFEA) must give approval before the procedure can go ahead.

The couple have said that they too will go to the US for treatment if they do not get permission to go ahead in this country.

In a similar case in the United States, a family had a test-tube baby to provide bone marrow for their six-year-old daughter who suffers from Fanconi anaemia.

Molly Nash received cells from her brother Adam's umbilical cord to help her fight the inherited disease.

The immature cells have the potential to develop into healthy bone marrow.