In developed countries, there are approximately 100 000
deliveries per year per 10 000 000 of the population.
The birth incidence of trisomy 21 is about 1 in 500, and
therefore in such a population the total number of affected
neonates is about 200.

A policy of screening on the basis of maternal age and
offering an invasive test to all women aged 35 years
or more would result in invasive testing in 15% of
the pregnancies (15 000), with consequent miscarriage
in 150, for the detection of 50% (100 of the 200) of
the trisomy 21 fetuses. The practice of (1) subjecting
all women aged 35 years or more to invasive testing;
(2) in those under the age of 35 years, carrying out a
series of additional sonographic and biochemical tests
in the first and second trimesters; (3) interpreting the
results of each screening test independently of each other
and (4) performing an invasive test in all women with
a screen-positive result, would potentially identify more
than 95% (190 of the 200) of the trisomic fetuses, but
this would be achieved by subjecting more than 40% of
the population (40 000) to invasive testing and causing
400 miscarriages.
A more rational approach is to carry out a screening
test at 11–14 weeks by combining maternal age with
sonographicmeasurement of fetalNT and maternal serum
measurement of free β-hCG and PAPP-A. In addition,
the fetal profile can be examined for the presence or
absence of the nasal bone. A detection rate of 95%
can potentially be achieved with an invasive testing rate
of about 2% (2000 pregnancies and 20 miscarriages).
It would then be irrational, both in terms of logistics
and economic cost, to subject the remaining 98 000
pregnancies to second-trimester serum biochemical testing
with the objective of identifying about 60–70% of the
remaining 10 cases of trisomy 21. Since all women should
be offered a second-trimester ultrasound scan to identify
major fetal abnormalities such as spina bifida and cardiac defects, the diagnosis of major and or minor defects,
including nasal bone hypoplasia, will potentially lead to
the detection of more than 70% of the remaining 10
cases of trisomy 21.