A couple from Leeds want to have a test-tube baby selected to help cure their other child of a devastating disease.
The Court of Appeal has now given them the go-ahead after the High Court called a halt to the procedure.
BBC News Online examines the facts and the ethical debate surrounding this technique.
Why does Zain Hashmi need help?
Zain has thalassaemia major, a rare blood disorder which is thought to affect 100,000 people worldwide.
Patients with this disorder are unable to make enough of a chemical vital to the construction of red blood cells.
These are the cells which carry oxygen around the body to keep tissues alive - a child with severe thalassaemia will become anaemic, fail to thrive, and, if left without treatment, can die in childhood.
The normal treatment is blood transfusions, given once every three or four weeks - these supply the patient with enough red blood cells to keep going.
However, over time, the level of iron in the body can build up to dangerous levels. From adulthood onwards, patients will need an extra drug to deal with this, and may have to wear a pump to infuse this slowly.
Another possibility is a bone marrow transplant - the existing bone marrow cells are destroyed using chemotherapy, then new, healthy ones are transplanted in to take over their job.
However, the technique requires finding a donor with an exact match, which is very hard to do.
What causes thalassaemia?
Thalassaemia is caused by an abnormality of the gene in humans which helps produce haemoglobin, the vital chemical.
People who are likely to carry the gene of thalassaemia are people with Mediterranean descent, for example Cyprus, Egypt, Greece, India, Italy, Lebanon, Malta, Middle East, Turkey and some parts of South East Asia.
It is possible for people to people to carry the gene, yet only develop a relatively mild version of the illness, which may go unnoticed for years.
How can IVF doctors help?
In IVF, human embryos are created outside of the body by mixing eggs and sperm together - making a so-called "test tube baby".
A couple of days later, best ones are selected and implanted into the woman, where a pregnancy will hopefully follow.
Having the embryos close at hand during this period allows a tiny amount of dna from each to be taken away and analysed for its genetic makeup.
In this way, doctors could not only pick an embryo which did not carry the thalassaemia gene, but also pick one which was otherwise a complete genetic match for Zain.
The embryo could develop into a child whose bone marrow would be the identical match needed for a bone marrow transplant.
The cells needed for the transplant would be taken from the umbilical cord of the newborn baby.
This technique is similar to other forms of pre-implantation genetic diagnosis (PGD), which picks out embryos likely to develop into babies with serious genetic diseases, such as muscular dystrophy.
Will it work?
Not necessarily. First, the experts must find embryos which are both free of the genetic defect, and which will produce a baby with matching umbilical cells.
This may mean that there are fewer good quality embryos than normal to re-implant into the woman, reducing her chances of successful pregnancy.
Even if a child is born, there is no guarantee that the child will be cured by the infusion of cord cells or bone marrow.
What is the ethical dilemma facing doctors?
Doctors have no qualms about using umbilical cord blood from a baby in this way.
However, it is the motives of the parents in creating a child expressly for this purpose that is troubling.
Critics have likened the concept to producing a baby simply to harvest "spare parts".
There are concerns over whether the parents actually want another baby, and whether that baby would be valued the same as his or her brother.
In this case, the umbilicus is obviously surplus to requirements, and no harm comes to the baby by its use, but ethical experts would be nervous about setting any ethical precedent about the use of specially-created body parts in this way.
Why was there a court fight over this?
The pro-life group CORE (Comment on Reproductive Ethics) are opposed to the treatment.
After the HFEA gave permission for the treatment to go ahead, they took the case to the High Court, arguing that the HFEA had no powers to grant permission for the procedure.
The High Court judge originally agreed with CORE, and called a halt to the procedure.
However, an application to the Court of Appeal was successful, and the ban was overturned in April.
What must couples wishing to do this do now?
The latest ruling still does not give couples and clinics carte blanche to carry out these procedures.
Each case - including that of the Hashmis, must be considered individually.
Applications to the HFEA are made via the clinic which wants to perform the procedure.
The HFEA says it will take into account the likely welfare of any new child - alongside the situation of the existing child.
However, only those couples whose child suffers from a small number of genetic disorders are covered by the ruling.
A couple whose child developed leukaemia are not covered by the ruling.