Biochemical and ultrasound screening for chromosomal abnormalities

The utility of ultrasound screening for Down syndrome should be judged by whether the pertinent markers are reliably obtained and by whether they efficiently discriminate between fetuses with Down syndrome and euploid fetuses. Furthermore, the markers must be reliable at a gestational age early enough to be clinically useful. Screening for characteristic congenital malformations will result in a low detection rate of Down syndrome, and many of these cases will be identified only after 24 weeks of gestation. The subtlety of many of the ultrasound findings of Down syndrome in the second trimester requires significant technical expertise if ultrasonographic screening is to be used. Investigation thus far has centered on identifying patients at increased risk of Down syndrome, and no data are available on how much a normal ultrasound might decrease the risk associated with advanced maternal age or abnormal biochemical screening. Biochemical screening is currently able to detect at least 60% of Down syndrome in a low-risk population. The ultimate value of ultrasound may be to help further define the risk assigned by age and biochemical screening, to provide each patient with an aggregate risk. The decision on whether or not to offer an amniocentesis could then be based on the findings of these various examinations, with the goal to improve the accuracy of age or biochemical screening alone. It remains for studies to be done to determine whether ultrasound can in fact decrease the prior risk, and for large enough series and formulas to be published that allow us to specifically define how to integrate the information from each screen.