Testing for Downs Syndrome and fetal abnormalities in early pregnancy

There have been a recent developments in testing for Down’s Syndrome (DS) and fetal abnormalities in early pregnancy. Up until recently the choice for testing (there is of course no compulsion to have any form of testing – many couples choose not to undergo DS screening tests) was between ultrasound/blood test (US) based screening and then proceeding to more invasive testing if indicated based on the results or proceeding directly to invasive testing without doing the US test.
Ultrasound based screening (Nuchal Transluscency or NT) involves a blood test performed on the mother at about 11 weeks to measure some proteins in her blood which are produced by the pregnancy followed by an US to measure the skin fold thickness at the back of the baby’s neck. These results in combination with the mother’s age are used to calculate the risk of the baby having not only DS but also a range of other significant abnormalities. This result provides a guide as the whether or not to proceed with further testing. Of all the women who undergo this test about 5% will be advised to consider further testing but only a very small proportion of these babies will have DS or another abnormality. The testing does not identify all babies with DS (about 5% of DS babies will not be identified by this testing approach). However as well as identifying the risk of DS this testing can also identify or give clues to a number of other potentially serious abnormalities in the baby.

Invasive testing is the next step if the the above test indicates a “high risk” result or for those women who choose to proceed directly to this type of test. This type of test involves taking a sample from inside the uterus, usually by passing a fine needle through the abdominal wall of the mother. The sample can be either the fluid that the baby is swimming in (amniocentesis done at about 15 weeks) or a sample from the placenta (CVS done at about 11 weeks). These tests are both highly accurate for DS and similar types of conditions as well as being able to identify many other less common genetic abnormalities. They provide a definite “yes” or “no” answer in regards to DS in the vast majority of cases. They do however both have a small risk of causing miscarriage (for CVS about 1:100 and for amniocentesis about 1:300).

The new test: NIPT (Non-invasive prenatal testing)
This test has recently become available. It is a blood test performed on the mother after 10 weeks of pregnancy. It identifies the DNA of the baby in the mother’s blood and therefore is able to identify DS without taking a sample from inside the uterus. This is a very exciting development. Before you get too excited here are a few facts about it
– turnaround time for the test is generally about 1 week
-it is expensive, about $500 or so. No medicare rebate
– in a small proportion of tests (< 2%) the test is not successful (there is not enough fetal DNA present to produce a result)
– the test result is highly reliable, if the test indicates that the baby does not have DS then this can be relied upon to a level of about 99.8%. If the test shows that the baby has DS, it very likely to be correct but there is a 1:1,000 chance that it will be wrong (ie baby is normal) and for this reason it is recommended that invasive testing be done for confirmation before pregnancy termination is performed.
-it identifies DS and DS like chromosome abnormalities but does not identify the range of genetic or other abnormalities identified by amniocentesis/CVS or detailed US. Does this matter? Probably yes. Depending on your age group, DS only represents about 50% of the serious abnormalities which could be diagnosed at this early stage of pregnancy.
Where does this new test fit in at this stage?
NIPT is the best of the non-invasive tests for finding DS, however it is not very good and finding many of the other significant abnormalities which could be diagnosed by US based screening.
It can be used as a followup for women who undergo US based screening (NT) and who have a high risk result as an alternative to invasive testing. I would see that as it’s main role at this stage. That is, giving women who have a high risk NT an alternative to invasive testing. Up until now the option has been either proceed to invasive testing (with the small risk of miscarriage) or do no further testing.

If you choose to do NIPT as your first test, you should probably also undergo US based screening in addition as there are a number of serious conditions other than DS which will not be diagnosed by NIPT but which are likely to be diagnosed by US based screening.

Advertisements