Down Syndrome Screening Test
The screening test for Down syndrome is part of the Maternal Serum Screening Test (MSS). The combined first trimester test is the most commonly offered test. This test is offered between 11 and 14 weeks gestation. The test includes a blood test to measure the levels of two naturally occuring proteins in the mother's blood and an ultrasound test to measure the amount of fluid in the back of the baby's neck. The results of the MMS will give the parents either a 'low-risk' or a 'high-risk' result.
The meanings of screening and diagnosis are different but they are often confused. Screening involves determining the risks or likelihood of a condition, whereas a diagnostic test will give a definite answer. Sometimes the results of a screening test might suggest the need for an invasive test in order to obtain a diagnosis, for example an amniocentesis for Down syndrome.
There are several variations of the Maternal Serum Screening (MSS) test and your local health service will determine the specific tests offered. The combined screening test is recommended (NICE 2008) between 11 weeks and 14 weeks, with the triple or quadruple test if you book in later, at 15-22 weeks.
Advantages of having the Maternal Serum Screening Test
- These tests do not harm your baby nor increase the risk of miscarriage.
- Screening tests give you a result on which to choose diagnostic testing or not.
Disadvantages of having the Maternal Serum Screening Test
- The process may make you very anxious.
- These tests will not tell you if your baby has Down syndrome.
- Serum Screening will miss 1 in 6 babies with Down syndrome.
- The detection rates vary with your age e.g. a 15 year old woman only has a 77% chance of the test detecting a baby with Down syndrome; a 30 year old has a 84% chance of the test detecting a baby with Down syndrome and a 49 year old has a 100% chance.
- The rates of having a false positive result i.e. being told that you have a high risk of having a baby with Down syndrome and then subsequently finding out that your baby does not have Down syndrome vary with age. Research has shown that a15 year old woman only has a 1.9% chance of a false-positive high-risk screening result and a 30 year old has a 4% chance of a false positive result. However, a 49 year old has a 67% chance.
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