Assessment of atypicality: an adjunct to prenatal screening for Down's syndrome that facilitates detection of other abnormalities.

Current software used for assessment of the risk of Down's syndrome may give misleading risk estimates if applied to other abnormalities. Often the abnormality is reflected in maternal serum alpha-fetoprotein and human chorionic gonadotrophin levels and is then translated into a low risk for Down's syndrome that may not be recognized as significantly atypical of normality. We regard this as a serious deficiency in the current Down's syndrome risk reporting algorithm, and suggest a modification that allows the problem to be overcome.