Freddie and Thomas Greenstreet were born at Guy’s Hospital, London, after they were examined as embryos using preimplantation genetic haplotyping (PGH), an improvement on earlier methods.

Their parents, Jim and Catherine Greenstreet, are both carriers of the gene for cystic fibrosis. They already had two children, one of whom, Lizzie, has the disease.

Embryos have been tested for some time using a technique called PGD — preimplantation genetic diagnosis — and the Greenstreet twins are the 100th birth at Guy’s using this method. But it depends on a precise knowledge of the gene responsible for the particular disease, and assumes that the mutation is the same for everybody affected. PGH, developed at Guy’s and St Thomas’ NHS Foundation Trust, is a better technique because it allows a wider range of genetic disorders to be tested for.

It relies on tests of various members of the family, both affected and unaffected, to determine a suite of genetic markers (the haplotype) that are inherited along with the condition.

Then a cell is taken from the IVF embryo and multiplied a million fold so that it can be tested for the presence of the same markers. The advantage of the technique is that it does not require a precise knowledge of the genes causing the problem in a family.

The technique is also useful for X-linked disorders, those caused by mutations in the X-chromosome. Girls have two X-chromosomes so can usually rely on one of them being normal, but boys have only one. If there is an abnormality — such as those causing haemophilia or muscular dystrophy — they have no spare to fall back on.

Until now, selection for X-linked disorders has simply meant throwing away male embryos and only implanting female ones. But PGH enables healthy male embryos to be identified.

Mrs Greenstreet 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, and for them to have the extra special honour of being Guy’s and St Thomas’ 100th PGD babies and the UK’s first PGH babies.”

Alison Lashwood, a consultant nurse in genetics and PGD, said: “It is fantastic to have helped so many couples who were carrying genes for serious genetic disorders to have the babies they may have thought they could never have.

“We are very proud of this achievement and, as one of the biggest and most advanced PGD centres in Europe, we hope we can help many more couples in the future.”

Professor Peter Braude, head of the department of women’s health at King’s College and clinical director of the PGD programme added: “PGD can make an enormous difference for some families. 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.”

GENE DEFECTS

• About 200 inherited defects, including the commonest form of cystic fibrosis, can already be identified in embryos using a technique called pre-implantation genetic diagnosis (PGD)
  
• When it was developed in 1989 it was allowed only for genes that always cause disease. Last year it was approved for a form of eye cancer that affects only 90 per cent of those who inherit a mutated gene
  
• Another screening test works by removing a single cell from a three-day-old embryo and checking it for aneuploidy, a set of disorders in which an abnormal number of chromosomes triggers miscarriage or genetic conditions such as Down’s syndrome

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