Screening for Down Syndrome

I spend more time with couples at their first antenatal visit explaining the maze of first trimester screening than I do discussing most other aspects of their ongoing pregnancy care. Why? Because it's a topic that couples want to know about. Downs syndrome is one of the most common chromosomal disorders and the most common cause of intellectual disability and we can screen for it - that is, we can do tests which direct us to which pregnancies are at a higher risk of Down Syndrome. Explaining these screning tests is like navigating a maze of information and it takes time.

The greatest risk factor for the conception of a Down Syndrome baby is maternal age - the age of the mother. The older you are - the greater the risk. Most women fall into the low risk age group of less than 35 but we know that a large number of Down Syndrome pregnancies are still conceived in this age group. There are also an increasing number of women older than age 35 that are delaying having their families for various reasons and therefore fall into the increased risk age group.

We can get a better idea as to which pregnancies are at increased risk of Down Syndrome by using a screening test. The standard screening test involves taking a blood sample from the mother at about 10 to 11 weeks to measure some hormones (PAPP-a and HCG). These hormone levels are then combined with a measurement taken of the thickness of the skin at the back of the baby’s neck (nuchal translucency - NT) which is measured at around 12 weeks gestation. All these figures are put into a computer program along with a number of other parameters and the resulting figure is your risk for having a Down Syndrome baby for this individual pregnancy. This combined screening test detects 85-90% of all Down Syndrome pregnancies. There is an approximately 5% false positive rate with this test - 5 out of every 100 tests indicate that the pregnancy is affected by Downs Syndrome but further testing indicates that the pregnancy is normal. The hormone assays can also give information on placental function. The 12 week ultrasound can also give an indication of other abnormalities of fetal anatomy which may require further testing. If the combined result is low risk (say, 1 in 1000) then we would generally say – "It’s all good, you don’t need to do any further testing" – with the understanding that there is still a 1 in 1000 chance of you having a Down Syndrome baby. If it is increased risk (say, 1 in 100 – the cut off for low/high risk generally being 1 in 300) then you have the option of either ignoring the test result (this is uncommon) or go on to have more invasive / definitive testing. This would generally consist of a Chorionic Villus Sampling (CVS). With this procedure a needle passes through the abdomen, through the uterus and into the placenta to sample some tissue. There is a roughly 1% risk of miscarriage. Rarely there is a problem with getting a sample. Rarer still, there is a risk of a wrong result.

Couples generally fall into one of three groups: Group 1 – want to know if they are having a Down Syndrome baby and would terminate if they have an affected pregnancy. Group 2 – having a Down Syndrome baby is no big deal / they will “take what they get” OR they would never terminate a pregnancy for various different reasons. Group 3 – never heard of the test, just get it because they think they should, without bothering to think about the roll on effect of an abnormal test result. I used to say that if you were in Group 2 then you should think very seriously about whether you really want to get the test. However, there is an increasing number of couples who would not necessarily terminate because of the result but would like to know in order to plan for the future. These couples usually opt for an amniocentesis if they go on to require full diagnostic results of the genetic makeup of the baby. Amniocentesis is done later than a CVS and has a much lower risk for miscarriage. It still involves a needle going into the uterus but fluid is collected rather than placenta tissue. First trimester screening has been somewhat complicated by the more recent availability of non-invasive prenatal testing (NIPT) where the baby's DNA is detected in the mothers blood. This sounds like something out of a Star Trek movie but through the miracle of modern technology the baby's DNA can be tested for chromosomal abnormalities. This test screens for abnormalities of chromosomes 13, 18 and 21 and the sex chromosomes X and Y. NIPT does not give any indication of other chromosomal abnormalities at this time (watch this space). NIPT is a better screening test for Down Syndrome than routine screening with greater detection of Down Syndrome foetuses and less false positives. However, the test is expensive (~$500) with no medicare rebate and IT IS STILL A SCREENING TEST. The test can be done from 10 weeks gestation. Any abnormalities must be confirmed by invasive testing (CVS or amniocentesis). Also, a 12 week ultrasound is still strongly recommended to check for any anatomical abnormalities of the developing fetus. In a perfect world, each woman who wished to would get all three tests - hormone test, NT and NIPT. At this time however, cost makes this difficult or impossible. We are currently in a situation where the development of technology has outpaced the development of protocols. Protocols for first trimester screening are still being developed for the different populations of expectant mothers in Australia. How NIPT is integrated into first trimester screening in Australia is yet to be determined.

My current recommendation to most couples is that they have the combined screening done as it gives information on not only your risk of Downs syndrome but also on placental function, anatomy, other chromosomal abnormalities, risk of preeclampsia AND risk of problems with foetal growth later in the pregnancy. If they are PARTICULARLY concerned about the possibility of Down syndrome then I suggest that they get the NIPT testing in addition to the routine screening.

Most Down Syndrome adults are able to lead full lives as valued and productive members of their families and the general community. However, Down Syndrome can be an extremely challenging diagnosis both pre- and post-natally. A small number of Down Syndrome children and adults can be severely intellectually handicapped with disabilities affecting other organ systems of the body.

A lot of couples ask me whether they should have the testing for Down Syndrome done or not. This is an area where I cannot guide couples as to what they should do. It's an intensely personal choice based on their own percieved risk, their beliefs and also - in a lot of cases - on their preexisting family structure.