GENETIC screening can be used to test for conditions such as cystic fibrosis, muscular dystrophy, haemophilia and Huntington's disease.

The technique of preimplantation Genetic Diagnosis (PGD) was developed in the late 1980s and involves testing a live embryo to see if it contains potentially 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 fertility experts can test for more than 60 conditions using PGD.

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

A single cell must first be removed from a three-day-old, eight-cell embryo using a very fine pipette. The cell is then analysed.

The main objection to the procedure is that it opens the door to a world of nightmarish possibilities. If embryos can be selected to be free of harmful genes, critics argue, who is to say they will never be screened for particular genetic traits that parents might desire or want to avoid? Alternatively, deaf or blind couples might want their disabilities passed on to their child.

Advocates of PGD insist there is no danger of it leading to a super-race of designer babies. They say careful use of PGD has the potential to eradicate serious inherited diseases that have plagued families for generations.