We have concerns over the proposal that screening for Down's Syndrome is offered in an unstructured manner, offering all possible options to women in the manner suggested by Alfirevic. We believe that screening should be offered and conducted according to guidelines which will help to ensure that screening programmes are worthwhile. In general, there should be no compromise over offering the most effective and safe method of screening that is affordable, while at the same time one can compromise on what test is performed, taking into account a patient's concerns and preferences. In other words, a programme could offer the integrated test, but if a woman was particularly concerned about receiving a screening result at 15 weeks instead of 11 weeks, she could have the first trimester combined test, provided she was aware of the associated reduced efficacy and safety of such screening. Similarly, if a woman was acutely anxious over the possibility of having a pregnancy with Down's Syndrome, however remote, an amniocentesis could be considered.
Offering invasive testing to all women regardless of risk would be expensive and associated with many fetal losses, whereas offering such invasive testing to screen positive women is less expensive, with many fewer fetal losses; one is only offering invasive testing to 1–5% of women instead of 100%. Women with screen-negative results are not routinely offered an amniocentesis, but they can have an estimate of their risk of having an affected pregnancy and if they are worried about this can discuss it with their obstetrician and request an amniocentesis. The purpose of screening is to identify as many affected pregnancies as possible while minimising the harm to women with unaffected pregnancies. Current screening tests can achieve high detection rates and low false-positive rates. Offering chorion villus sampling or amniocentesis to all women will tend to maximise the harm.