Screening for Down's syndrome. Biochemical screening offers advantages.

Editor—We were interested in the findings of Howe et al on screening for Down's syndrome in Southampton and their suggestion that the benefits of mid-trimester serum screening have been overrated.1 The Women's Centre at the John Radcliffe Hospital in Oxford has a similar population in terms of the number of deliveries and the percentage of women having prenatal karyotyping (7%) and has a similar screening policy for Down's syndrome: amniocentesis offered to women aged ⩾35 at expected date of delivery; no serum or nuchal translucency screening through the NHS (some women organise this privately). The population at the Women's Centre has a higher distribution in maternal age (16% aged ⩾35, compared with 10% in Southampton). We performed a similar analysis on 78 cases of Down's syndrome in 1993-8 in which the fetus was alive at the time of scan (table). Forty three (55%) cases were diagnosed prenatally before 24 weeks' gestation; in mothers aged under 35 years this rate was 41%. Of the 35 cases not prenatally diagnosed before 24 weeks' gestation, eight were in babies born to women aged ⩾35 years (eligible for amniocentesis). The increase in prenatal detection in Oxford in 1993-8 was mostly due to suspicion at the anomaly scan. However, the detection rate in Oxford is lower than that reported in Southampton, despite the higher maternal age in Oxford. We agree that there are advantages to using the anomaly scan as a screening tool for Down's syndrome—it is more economical, there is a single intervention, and other chromosomal abnormalities are more readily detected—but there are also disadvantages. For example, most women are not aware that the anomaly scan may lead to the detection of Down's syndrome and a relatively late diagnosis. Biochemical screening would have some advantages: earlier diagnosis, more informed consent (with appropriate pretest counselling), and a higher detection rate for a lower false positive rate (and hence fewer normal babies lost through miscarriage). We agree with Howe et al that it is important to obtain evidence of the effectiveness of new screening methods2 and to compare them with current practice before their widespread introduction.