1.7 Calculating the risk factor
The results of each of the screening tests are compared with the population median values for unaffected pregnancies to produce risk modifiers or likelihood ratios. Each of these is used to modify the mother's risk of carrying a fetus with DS due to her age alone (known as the 'a priori risk') to calculate a combined risk. In the UK, the National Screening Committee (NSC) standards for first trimester combined screening state that pregnant women with a risk higher than 1 in 150 should be offered a diagnostic test. At this cut-off, the test should detect at least 80% of DS-affected pregnancies with an approximately 3% false positive rate.
There are several commercial computer-based algorithms available for calculating the revised risk factor for DS based on the outcome of the screening tests. For the first trimester test, these programs take a nuchal fold measurement of the fetus, and the MoM values for PAPP-A and hCG plus the maternal age to produce a revised risk factor. For the second trimester, the MoM values for the four maternal serum tests are obtained. The values are fed into the computer program which then produces a revised risk factor, which is determined from the risk factor derived from the known incidence of DS with maternal age. More sophisticated programs also take into account the weight, ethnicity, whether the woman smokes or has diabetes, and whether it is a multiple pregnancy; all of these factors either change the risk or affect the test results.
A pregnant woman aged 35 has a risk of 1 in 250 of having a child with DS based on age alone. She has a scan which shows an NT measurement of 3.0 mm and her serum has a MoM for PAPP-A of 0.4 and for hCG of 1.7. What do these data imply?
These measurements show that there is an increased risk, so the risk will go from 1 in 250 (which can also be written as 1 : 250) to whichever risk the computer calculates. This revised risk may be 1 in 50, but it also depends on the other factors mentioned above.
When a pregnant woman receives the results of her screening tests, and if her risk is raised, she may be offered a diagnostic test for DS or other chromosomal abnormalities. The risk of having a baby with DS may be quoted as being 1 in 100, whereas the risk of miscarrying a fetus after the invasive test may also be quoted as 1 in 100. How does the expectant mother weigh up these risks and decide whether to have an amniocentesis test? This very difficult decision is the subject of the next section.