The first UK babies have been born after a pioneering embryo test.
The Greenstreets with their four children
Freddie and Thomas Greenstreet were born two weeks ago.
Their parents Catherine and Jim already have a daughter affected by cystic fibrosis (CF).
The test, called pre-implantation genetic haplotyping, looks for a genetic 'fingerprint' of a condition, such as CF, sickle cell disease, Duchenne muscular dystrophy, Huntington's disease and spinal muscular atrophy.
It looks at the whole DNA of a cell from an embryo, rather than focusing on a specific mutation in one gene, making it quicker to spot diseases.
The researchers also look at genetic clues for the disease in the mother and father. If a certain number are present, it suggests the embryo is affected.
Both Catherine and her husband Jim, from Chiswick, carry the CF gene.
They already have twins, Lizzie and William aged five. Lizzie has CF.
Doctors at Guy's and St Thomas' Hospital in London tested the Greenstreets' embryos to identify those which were unaffected by the condition.
They then implanted healthy embryos.
Catherine said: "The whole family is very excited about the arrival of Freddie and Thomas.
"At one point we didn't think that we would be able to have any more children, so it is quite something to have two healthy, happy babies."
Professor Peter Braude, head of the department of women's health at King's College and clinical director of the pre-implantation genetic diagnosis (PGD) programme said: "PGD can make an enormous difference for some families who have repeatedly miscarried, lost children through serious genetic disease, or are living with a child suffering from a serious genetic disease.
"For these couples it allows them the best chance of avoiding a serious genetic disorder being diagnosed in pregnancy and, in some cases, the need for a termination of pregnancy."
Rosie Barnes, chief executive of the Cystic Fibrosis Trust, said: "Couples who know they are at risk from having a child with CF are often those who already have a child with CF, so know they are both carriers of the defective gene.
"Understandably they think carefully before having another child.
"Knowing that CF is a life-threatening disease and that living with the daily burden of physiotherapy, oral, nebulised and intravenous antibiotics places a huge strain on all involved, many do not wish to bring another child with CF into the world.
"People should always be given the right to make their own informed choice, and this new development in PGD extends this choice, offering the option of having a healthy child without resorting to an abortion, to carriers of less common mutations of the CF gene."
But Josephine Quintavalle, of Comment on Reproductive Ethics, said: "It is not about the babies who have been born. It's about the babies who have not been allowed to be born.
"It sends a particularly bad message to children with cystic fibrosis - that society would prefer that they had not been born."