Downs syndrome screening in pregnancy

Everyone has 23 pairs of chromosomes. These contain the genetic codes that make up our genome and individual characteristics. There are many known changes that can occur on the chromosomes that can lead to a variety of abnormalities. Some are lethal and lead to miscarriages or stillbirth, others are compatible with life. However, in many babies born with such abnormalities, there are significant consequences to their well-being. Some may have mental retardation, multiple organ abnormalities like abnormal kidneys or hearts and so on.

The most commonly spoken one is Downs Syndrome. It is due to an excess of Chromosome 21. This can be due to an additional chromosome ether in part or in whole. Where there is an additional chromosome, it is called Trisomy 21. However, where an additional part of the chromosome is found      (as opposed to a whole chromosome), this is due to a translocation. Irrespective, the baby will be affected by the features of Downs Syndrome.

The reason why Downs Syndrome is commonly spoken of, is because the risk of it being present in the baby increases with maternal age. Whilst this risk applies to all chromosome abnormalities, Downs is the most common one. Hence the focus on this chromosome.

There are many tests used to try and predict the risk of Downs Syndrome in the baby. They range from the use of:

  1. Maternal age: With increasing age of the mother, it is well recorded that the risk of Downs Syndrome also increases. Unfortunately is it not an accurate screen. So it has been replaced over time with better screening techniques:
  2. Ultrasound alone: For a while this gave us a somewhat better indicator of Downs Syndrome risk than age alone. Usually performed at the 12 – 13 week mark of pregnancy, attempts to look for abnormalities in the thickness of the neck, presence or absence of other abnormalities led to an improvement in detection rates. Still not all that good.
  3. Combined testing with blood proteins and hormones significantly improved the ability to predict risk of babies being affected. This is now the standard test worldwide. It involves taking blood from the mother at 10 weeks pregnancy and the levels of the hormones and proteins are then combined with the ultrasound at 12 – 13 weeks looking at the thickness of the back of the neck and also the absence or presence of the nasal bone. It looks for abnormalities in Chromosome 13, 18 and 21.An additional result obtained from the Combined testing is the ability to check the level of a protein called Papp A which is derived from the placenta also gives valuable information on the likelihood of growth problems in the pregnancy for the baby. It does reflect placental function and abnormally low values may indicate, amongst other things, an increased risk of poor foetal growth. Knowing this, allows procedures to be instigated to minimise this risk.
  1. NIPT: This is the new kid on the block. It is also called Non Invasive Prenatal Testing. Using blood again from the mother at 10 weeks of pregnancy, it tries to pick up genetic material from the placenta. This is not the baby’s genetics but using computer programming, it can help to identify at risk babies more accurately than the Combined testing. Current use allows it to look at Chromosome 13, 18, 21, X and Y. (X and Y are the sex chromosomes) The accuracy can be as high at 99.4% for Downs Syndrome but usually is less for the others.Whilst exciting, it does not measure Papp A and so loses out in detecting what is becoming increasingly recognized as an important check for pregnancy risk. In addition there is a cost of about $500.00 which is non rebateable from Medicare. An ultrasound at 12 – 13 weeks is still required to be performed.

    The inability to have Papp A checked can be overcome. These days I also recommend that all women should have a screen for the risk of Pre-Eclampsia. This is the risk of a woman developing high blood pressure ( Pre-eclampsia) during the pregnancy. This screen is a combination of a blood testing and the ultrasound at 12 – 13 weeks. One of the chemicals tested for in this screen is Papp A.

    Another advantage is that with  NIPT, there is  an additional screen for abnormalities on the 22 q11.2 chromosome. This causes a condition called DiGeorge Syndrome. This can result in a variety of different conditions that can vary from minor to very major problems. it is in my opinion an important genetic abnormality to detect.

  2. If by chance you miss the deadlines for the above testing, we can still test for Downs Syndrome after the 14 week mark of the pregnancy. Called the Second Trimester Screen, it is again a blood test looking a different hormones and proteins in the maternal blood stream. Its accuracy is less than the Combined screen or NIPT, but can still be useful

It is important to understand that these are all screening tests only. They cannot tell you if your baby has or does not have Downs Syndrome or the other chromosomes tested for. In addition, they are limited to these chromosomes only.

If by chance there is a high risk result, then the next test will by necessity currently be an invasive test.  There are two tests available:     Chorionic villus sampling (CVS) or amniocentesis
Chorionic villus sampling (CVS) involves putting a needle through the tummy into the womb and taking a sample of the placental tissue to do chromosomal studies. Whilst it can be done any time after 11 weeks in the pregnancy, it does carry a high risk of miscarriage compared to an amniocentesis. Occasionally the results may be difficult to interpret at it is the placental tissue that is being sampled. There can be abnormal results due to placental abnormalities. The risk of this is not high, but it does exist.

Amniocentesis: This is the traditional method of checking the baby’s chromosomes. It involves putting a needle through the tummy into the womb and taking a sample of the fluid around the baby to do chromosomal studies. Within the fluid usually are cells from shed from the baby itself and it is these cells that are tested. Thus it provides a very accurate chromosome result and risk of miscarriage is lower than CVS. However it is performed much later in pregnancy – usually after 15 weeks. So the results come later and if an abortion is decided upon, it will usually be by inducing a labour.

The type of testing used will depend on the individual couple, the stage of their pregnancy and also their level of anxiety. No one test suits everyone and so I normally offer detailed counselling when I first see the couple.