When the first studies were published reporting the detection rate for Down syndrome using nuchal translucency measurements combined with maternal serum screening, investigators reported an overall detection rate for Down syndrome for women of all maternal ages as well as an average false-positive rate. The false-positive rate is the percent of women who are informed that their risk for Down syndrome is high and should undergo an invasive test (chorionic villus sampling or amniocentesis) but who have a normal fetus. The following lists examples from several studies.
Spencer et al, 2003
Detection Rate: 92%
False Positive Rate: 5.2%
Detection Rate: 78%
False Positive Rate:5%
Detection Rate: 82% to 87% (Depending upon gestational age the test was performed)
False-Positive Rate: 5%
The problem with this approach is that the detection rate for Down syndrome and the false-positive rate vary as a function of maternal age. Using 1st trimester nuchal translucency screening combined with measuring free beta-hCG and PAPP-A, the detection rate for Down syndrome increases as the maternal age increases. In the graph below this text you can see that at a maternal age of 16, the detection rate for Down syndrome is 77%. At age 36 it is 91%. At age 42 it is 98%. However, to increase the detection rate the number of women who are informed that their test is abnormal (false-positive rate) but who have a normal fetus, increases with maternal age.
Since the detection rate is not the same for all maternal ages, Genetic Ultrasound is a useful test to perform in the second trimester of pregnancy since it identifies over 95% of Down syndrome fetuses.
This graph compares the detection rate for Down syndrome and the false-positive rate, based upon maternal age. As the false-positive rate increases as a function of maternal age, more women will undergo invasive testing (CVS or Amniocentesis). (Specer K, Age related detection and false positive rates when screening for Down's Syndrome in first trimester using nuchal translucency and maternal serum free beta hCG and PAPP-A. British Journal of Obstetrics and Gynecology. 108:1043-46, 2001)