Women whose pregnancies are at increased risk of
Down’s Syndrome are likely to have a similar risk to other
pregnant women of having a baby with a structural chromosome
abnormality. The World Health Organization has
drawn up criteria for justifying screening programmes28;
full karyotyping, in the absence of fetal abnormality on
ultrasound, does not fulfil these criteria. Replacing prenatal
karyotyping with molecular trisomy testing for women
at risk of Down’s Syndrome would allow substantial financial
savings, the provision of rapid and unambiguous
results and the avoidance of anxiety and potentially unnecessary
terminations of pregnancy. Clearly, it is important to
have a figure that indicates the prevalence of significant
chromosome abnormalities in these pregnancies. A pessimistic
assumption would be that all those in the uncertain
prognosis group which were terminated, or for which we
have no outcome information, had abnormal fetuses (in addition,
a very few of those in the good prognosis group may
also have an adverse outcome); this assumption would lead
to a figure of 1 in 833 pregnancies with undetected chromosome
abnormalities. However, it is likely that many of
these pregnancies had a normal outcome, and therefore,
that close to 1 in 1600 pregnancies referred primarily for
Down’s Syndrome testing will have a chromosome abnormality
which will remain undetected, and, unless the
pregnancy miscarry, or fetal malformations are found at
ultrasound anomaly scan, will result in a liveborn infant
with unexpected significant clinical problems.