Charlotte Mariethoz loves her food. Curry is a favourite, but so are avocados, sauteed courgettes and prawns. Her sophisticated palette also appreciates such delights as braised venison and omelette aux fines herbes.

Her insatiable appetite requires no fewer than four square meals a day, plus numerous snacks. Despite consuming all these calories, she is seriously underweight. She has to wear a nasal-gastric feeding tube so extra nutrients can be given. At 21 months, she weighs just 15lb, the same as many three-month-old babies. The pink dress she is wearing today is meant for a girl aged three to six months.

“My cousin has a son who is three months old and already heavier than Charlotte”, says Catherine, her mother. “People are surprised to see her walking around because they think she is so young.”

Charlotte suffers from a rare blood disorder, diamond blackfan anaemia (DBA). So rare is the condition, whereby her bone marrow fails to create red blood cells or haemoglobin, that only three babies are born a year in Britain with the complaint. It is incurable and Charlotte has a life expectancy of only 20 to 30 years.

All that changed 10 days ago, when the Mariethozes, who live in Leicester, became the latest family in Britain permitted to create a “saviour sibling” to help cure her. To survive, she needs a stem cell transplant from a donor sibling who is a perfect genetic match.

The government’s fertility watchdog, the Human Fertilisation and Embryology Authority (HFEA), ruled that her parents may use a controversial screening procedure known as pre-implantation genetic diagnosis (PGD). This involves embryo selection: taking one or two cells from an embryo when it is around three days old. If the tissue types match, that embryo will be implanted in Catherine’s womb so it can develop. Once the baby is born, stem cells are removed from its umbilical cord, stored for four to six months, and then transplanted into Charlotte. The idea is to kick-start her own haemoglobin.

They are not the only family to benefit from embryo selection. Last week, in a landmark ruling, the HFEA allowed families with a history of cancer to use PGD. This means that a woman who knows she is carrying the BRCA1 or BRCA2 gene, responsible for breast cancer, can now screen her embryos for the faulty gene.

Catherine knows how lucky she is to have this chance of giving Charlotte a normal life. The alternative would be for her to try to have another child naturally, but the odds against getting a perfect match are four to one. At 43, she does not have time for trial and error, but tries to quell such disquieting thoughts. “I am a positive person”, she says. “I can’t go into something like IVF unless I think it’s going to work.”

Her treatment will be carried out by Dr Simon Fishel at a private clinic in Nottingham. He is one of the country’s leading experts, having assisted in the creation of Britain’s first test-tube baby, Louise Brown, in 1978.

Britain's first “designer” baby, Jodie Fletcher, was born in Belfast last July after her parents won a battle to overturn a ban on the technique. Her elder brother Joshua was also a DBA sufferer. “So far there has not been a successful birth to a couple who have had the treatment all the way through in England,” says Fishel. The Mariethoz baby would be the first.

The HFEA changed its policy after other couples had been forced to travel abroad to create a designer baby. Two years ago, Michelle and Jayson Whitaker from Derbyshire, sought treatment in America. Like Charlotte and Joshua, Charlie Whitaker, their son, suffered from DBA. Charlie received stem cells from his new brother, Jamie, and was cured.

Despite such success stories, PGD remains controversial. Pressure groups, including the pro-life lobby, warn of potential abuses, claiming babies can be selected on the basis of their eye colour or sex. They are also concerned about the pressure on the saviour sibling if the treatment does not save their elder sibling.

For Catherine, the ethics are simple: “This technique is the only cure . . . All we are doing is making sure that the second child — which we would have had anyway — is going to be a match.”

Without stem cell treatment, Charlotte could soon need steroids and more blood transfusions. She has already undergone eight in her short life. And, unlike many childhood cancers, which are increasingly curable, DBA is a great unknown. Symptoms include a cleft palate, among others, and stunted growth.

“Doctors have told us that Charlotte can expect to grow to only 4ft 7in,” she says. That, however, is the least of their worries as they battle to keep her free from infections.

During her pregnancy, Catherine was told to expect a large baby. “I thought I heard my husband say ‘4kg’,” says Catherine. In the event Charlotte weighed in at 4lb 7oz. She was listless and wouldn’t feed. Over the next few weeks she underwent a battery of tests at the Leicester Royal infirmary, doctors confirming their diagnosis in January 2005.

Inevitably the illness has affected their relationship. “It’s a strain”, says Catherine, a marketing manager. “It is difficult for it not to consume you.” She and Charlie, a chef, take it in turns to be with Charlotte in hospital. They have been married three years and have yet to enjoy a family holiday. “We have never been able to plan anything because we don’t know how well Charlotte is going to be.”

Holidays, in any case, are a luxury. First they must find about £40,000 for IVF. They have set up a website to raise money.

Catherine believes there is a strong economic argument for the health service to fund such procedures. “Treating Charlotte has already cost the NHS thousands. Some doctors think the costs during her lifetime could add up to £2m depending on treatment and drugs. I think IVF could be a cheaper option for them.”

Such are the harsh realities for this ordinary, middle-income Midlands family in a world where even cancer might be easier to contemplate, and where conception comes with a price tag.

For more information, visit www.charlotteschallengeappealfund.co.uk

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