Two leading London consultant obstetricians deplored the use of chorion villus sampling the new, earlier screening test for Down's syndrome on older women of no increased genetic risk.

Writing in the British Medical Journal, Richard Penketh and J. Malcolm Pearce of St. George's Hospital in London labelled the test "questionable" on any women, even those over 35, whose families do not have genetic disorders or a predisposition to Down's syndrome or spina bifida.Chorion villus sampling has been hailed as an improvement over the standard screening for Down's syndrome, amniocentesis, because it can be performed between eight and l2 weeks of pregnancy. It involves removing a small tissue sample from the chorion villi, where the embryo is joined to the placenta, either by going through the vagina or through the walls of the stomach. The sample is used to check for abnormalities such as Down's syndrome.

Those families wishing to abort a foetus shown to have Down's could do so in the first trimester of pregnancy.

Penketh and Pearce pointed out that the new test carries a higher risk of miscarriage and a higher level of false positives than amniocentesis (anywhere from 2 to 30 per cent, depending on the study). When a chorion villus test comes back positive, "in most cases this still means undergoing amniocentesis to confirm a foetal chromosome state", they said. The authors believe there is a l per cent chance of such women needing to go through both invasive procedures, which further increase the likelihood of producing a miscarriage.

The test also increases your chances of having an abortion, claim the authors. Because it is conducted so early, it will pick up foetuses with chromosomal abnormalities, "many of which would abort naturally," they said.