Preimplantation Genetic Diagnosis (PGD) was developed in the late 1980s and involves testing a live embryo to see if it contains harmful genetic abnormalities.

The embryo can then be used to produce a pregnancy, or discarded.

At the beginning, the number of disorders it was possible to identify was extremely limited.

Today in the UK there are more than 60 conditions fertility experts can test for using PGD.

They include a large number of "fully penetrant" problems which will inevitably develop in people who inherit a particular genetic defect. Examples include cystic fibrosis, muscular dystrophy, haemophilia and Huntington's disease.

Since 2006, the Human Fertilisation and Embryology Authority, which licenses PGD, has also allowed the selection of embryos free of faulty genes linked to breast, ovarian and bowel cancers.

The decision is controversial because these conditions may or may not develop in affected individuals; they are are not "fully penetrant". Also, if caught at an early stage, they can be cured.

PGD sounds a simple process but is complex and difficult, requiring sophisticated technology and high levels of skill. For this reason only a handful of fertility clinics in the UK are currently licensed to carry out the procedure.

Both single gene mutations and chromosomal abnormalities that affect whole "packages" of DNA can be detected with PGD.

PGD is carried out as an extra stage of In-Vitro Fertilisation (IVF).

A single cell must first be removed from a pinhead-sized, three-day-old, eight-cell embryo. The cell is drawn through an opening in the embryo's protective outer "shell" with a very fine pipette. This requires a high level of precision, since the cell is too small to see with the naked eye.

The cell is then analysed for genetic disorders. Three different methods are employed.

Fish (fluorescence in situ hybridisation) uses fluorescent probes which locate and bind to specific chromosomes; PCR (polymerase chain reaction) amplifies fragments of selected DNA extracted from a cell's nucleus to detect single gene defects; and haplotyping relies on DNA fingerprinting to identify chromosomes carrying affected genes.

Once a number of embryos have been tested, those free of the genetic abnormality can be set aside.

One or two of the healthy embryos are then selected for implantation into the mother's womb. Any other unaffected embryos can be deep frozen for future use, while those found to be defective are allowed to perish.