The Open University since 2006
Alternatively you can skip the navigation by pressing 'Enter'.
Full Steam Ahead: Episode sixThursday, 25th August 2016 20:00 - BBC TwoThe final episode of Full Steam Ahead looks at how more free time and rail transport allowed Victorians to travel... Read more: Full Steam Ahead: Episode six
Life Story: First stepsAvailable until Sunday, 25th September 2016 00:50Each generation's greatest challenge - to ensure the next generation thrives. Read more: Life Story: First steps
Hidden histories: Britain's oldest family businessesUncover the extraordinary history of three of Britain’s longest-running family businesses with... Watch now: Hidden histories: Britain's oldest family businesses
Full Steam AheadIt’s Full Steam Ahead for historians Ruth Goodman, Alex Langlands and Peter Ginn as they bring... Read more: Full Steam Ahead
Internet of everythingThe internet of everything (IoE) is the networked connection of people, process, data and things.... Try: Internet of everything now
An introduction to music theoryGain an understanding of the basic building blocks of musical theory and notation. This free... Try: An introduction to music theory now
Downs syndrome is the condition that is screened for the most in pregnant women because of the increased risk with the increasing age of mothers. This free course, Detecting Downs syndrome in the unborn fetus, describes several bioanalytical tests, and how these are used in conjunction with scans to screen and diagnose the condition.
After studying this course, you should be able to:
- explain the differences between screening and diagnosis
- identify the main physical and biological techniques used in screening for Down's syndrome
- appreciate the principles of assessing risk based on new information and how it is related to obtaining a revised risk factor for Down's syndrome
- assess the risks and benefits of diagnostic procedures for Down's syndrome such as amniocentesis and appreciate the ethical issues associated with screening and diagnosis
- understand the polymerase chain reaction technique.
- Learning outcomes
- 1 Screening the fetus
- 1.1 What is Down's syndrome?
- 1.2 Screening for DS
- 1.3 Screening in the first trimester of pregnancy
- 1.4 Ultrasound imaging
- 1.5 Biochemical first trimester tests for DS
- 1.6 Screening tests in the second trimester
- Current section: 1.7 Calculating the risk factor
- 2 Balancing the risk with ethical issues related to Down's syndrome
- 3 Diagnosing Down's syndrome
- Keep on learning
Study this free course
Enrol to access the full course, get recognition for the skills you learn, track your progress and on completion gain a statement of participation to demonstrate your learning to others. Make your learning visible!
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.
This free course includes adapted extracts from an Open University course which is no longer available to new students. If you found this interesting you could explore more free Across the Sciences courses or view the range of currently available OU Across the Sciences courses.
Copyright & revisions
Originally published: Tuesday, 22nd March 2016
Last updated on: Tuesday, 22nd March 2016
- Creative-Commons: The Open University is proud to release this free course under a Creative Commons licence. However, any third-party materials featured within it are used with permission and are not ours to give away. These materials are not subject to the Creative Commons licence. See terms and conditions. Full details can be found in the Acknowledgements and our FAQs section.
- This site has Copy Reuse Tracking enabled - see our FAQs for more information.
If you enjoyed this, why not follow a feed to find out when we have new things like it? Choose an RSS feed from the list below. (Don't know what to do with RSS feeds?)
Remember, you can also make your own, personal feed by combining tags from around OpenLearn.
All our alternative formats are free for you to download, for more information about the different formats we offer please see our FAQs. The most frequently used are Word (for accessibility), PDF (for print) and ePub and Kindle to download to eReaders*.
- Word (1.2 MB)
- PDF (16.6 MB)
- ePub 3.0 (34.4 MB)
- ePub 2.0 (1.5 MB)
- Kindle (680 KB)
- RSS (329 KB)
- HTML (76.7 MB)
- SCORM (76.7 MB)
- OUXML Package (39 KB)
- OUXML File (131 KB)
- IMS Common cartridge
- Moodle backup (60.6 MB)
*Please note you will need an ePub and Mobi reader for these formats.