The rapid changes in prenatal screening and diagnostic techniques bring new challenges. Alberry et al1 discuss the ‘unintended consequences’ of recent national guidance for anomaly screening and invasive testing from the point of view of a regional fetal medicine unit.

Developments in Trisomy 21 (Down Syndrome) screening have sought to increase sensitivity and specificity of screening tests. From the point of view of the individual woman, this means that if she chooses to participate in screening, she will have a high chance of detection of Down Syndrome, and a low chance that a subsequent invasive diagnostic test will be recommended when her baby is unaffected, since invasive testing carries a low but measurable risk of fetal death. In screening programme terms, the relevant parameters for programme participants are the Down Syndrome detection rate and the invasive test rate, and national guidance determines the ‘optimal’ balance between these two, which changes as the sensitivity and specificity of the tests are improved so that a higher detection rate can be achieved with a lower invasive testing rate. Soon, guidance may change again as the new non-invasive test using cell-free fetal DNA in maternal blood is introduced, initially as a second layer of screening for high risk women,2 being highly sensitive (>98%) and highly specific (>99.5%) but not yet diagnostic.

Trisomy 21 accounts for roughly half of all chromosomal abnormalities diagnosed in fetal and infant life in recent years in the UK and the rest of Europe.3 A …