The newest type of ultrasound scan the nuchal translucency scan is not much more accurate than a normal scan for detecting Downs' syndrome. For the last few years, women have been promised that this early scan, which measures the thickness of the nuchal fold at the back of the foetal neck, will give more accurate results. But research from King's College in London on more than 96,000 pregnancies has found that the "gee whiz" scan only picks up around 80 per cent of affected pregnancies in women known to be at high risk similar results to those obtained with a conventional scan.
According to the authors, around 30 invasive tests would have to be performed to find one affected foetus. Early detection has other drawbacks, such as identifying foetuses which may miscarry naturally. In addition, about 40 per cent of Downs' foetuses die between 12 weeks and term. Early detection puts the onus on women to make difficult and possibly unnecessary decisions about termination. In the meantime, the long term effects of exposing a very young foetus to increasingly powerful ultrasonic beams have yet to be studied (Lancet, 1998; 352: 343-6).