Aneuploidy is an accident of growth, in which dividing cells produce an abnormal number of chromosomes.

In most cases, these cause spontaneous miscarriage - the mother never knows she was pregnant - but in a smaller number of cases, a baby may be born with Down's syndrome or other conditions.

Aneuploidy could be responsible for up to 20% of all birth defects. And women over 35 are increasingly at risk.

Aneuploidy screening is yet another way to offer hope to couples who take IVF treatment - and to make sure of healthy children when a "test tube" embryo is successfully implanted. IVF researchers usually make up to 10 embryos, in the hope of finding at least two which can be implanted. But at that stage, abnormal embryos look pretty much like normal ones, so the new screening technique has become another weapon in the researcher's arsenal.

A female egg is fertilised with male sperm and then allowed to divide and grow in a medium until it becomes a ball of about eight cells, hardly bigger than a full stop in this newspaper. Using a powerful microscope and a very fine pipette, experts gently detach one of the cells, and stain it a series of fluorescent coloured dyes each of which is taken up by just one particular chromosome.

With the help of powerful magnification, experts can then see whether there are any extra copies of a chromosome, and which. The technique is aimed mainly at chromosomes 13, 18, 21. An embryo with three copies of any of these chromosomes could become a child that survived beyond birth with either Patau's syndrome, Edward's syndrome or Down's.

The process takes a few hours. However, it is possible for some cells to be normal, some aneuploid, so the scientists might test a second cell.

At that stage, the embryo can afford to sacrifice two of its cells and still develop normally.

Aneuploidy is not a genetic defect, it is an accident of cell machinery that becomes more likely with increasing years. But the technique is likely to complement the growth of genetic screening for inherited diseases caused by mutations in specific genes. There are more than 4,000 such "single gene" disorders.