interpretation of results from screening tests in Down's syndrome

Last reviewed 01/2018

Interpretation of results from initial screening procedures:

The results of one or both initial screening procedures are entered into a software programme which calculates the risk for a woman of having a child with Down's syndrome at her present age. This risk is calculated in relation to that of the population covered by the programme. The levels of risk associated with having a Down's syndrome pregnancy in relation to a woman's age are shown in the table below:

 

Woman's age (years)	Risk as a ratio	%Risk
Below 20	1:1600	0.067
20	1:1500	0.066
30	1:800	0.125
35	1:270	0.37
40	1:100	1.0
45 and over	1.50 and greater	2.0

 

If the risk is greater than 1 in 250, it is judged to be within the higher risk category. In such cases women will be offered a diagnostic test such as amniocentesis or chorionic villus sampling (CVS).

Presently about 5% of all pregnant women undergoing screening will have a high-risk result, and need to be offered a follow-on diagnostic procedure. However, the percentage of women who undergo such procedures is slightly lower than 5% because not all women who have a high-risk result choose to have an invasive procedure. It is one of the aims of the programme to lower the number of women who are offered an invasive diagnostic test by improving the specificity of the screening tests. To ensure interpretation of results from the initial screening procedures is as accurate as possible, it is essential to establish the date of the pregnancy. This need for accuracy underlines the importance of women having an early ultrasound scan to date the pregnancy

For more information then see www.screening.nhs.uk/an