What is Down’s syndrome?
It is very important to remember that most babies are born without any problems, but about 1 in every 700 babies will have Down’s syndrome. This is a genetic condition. Our DNA, a complex chemical that codes our genetic information, determines all the characteristics that makes us who we are. Sections of DNA are packed together to make a gene and several thousand genes are then coiled up to make up a chromosome. Every cell in our body has 46 of these chromosomes, 23 coming from our mother and 23 from our father. A person with Down’s syndrome has 47 chromosomes instead; they have an extra copy of chromosome 21 and this causes a genetic imbalance that leads to a variable amount of both physical disabilities and learning difficulties. Nothing you do can cause it or prevent it. It is just something that happens at conception and potentially anyone could have an affected baby, but it is more common in older mothers.
Why do we screen for Down’s syndrome?
Over the last few years, there have been lots of advances in how to screen for Down’s syndrome. In most cases the tests will reassure you that it very unlikely that your baby is affected, but just occasionally the test may indicate you are at high-risk, although it is important to remember that this still doesn’t mean that the baby has Down’s syndrome.
For some parents knowing that their baby has Down’s syndrome wouldn’t affect their decision to continue with the pregnancy, but it allows them the opportunity to plan and learn more about the condition before their baby is born. However other parents would choose not to continue with the pregnancy. It is a very personal decision, and a very difficult one. The medical team looking after you will give you as much information and support as you need to help you decide what to do. But before doing this you need to be absolutely sure that the baby is affected through an “invasive test”.
These invasive tests do give you a definite answer as to whether your baby is affected or not, but they involve taking a sample of tissue directly from the placenta (chorionic villus sampling or CVS) or a small amount of fluid from around the baby (an amniocentesis) and both of these tests can cause a miscarriage, even if the baby is healthy. This happens rarely, but often enough to mean that they should only be done if there is a high suspicion that the baby may have a problem.
Until recently the best test to assess whether you are at high-risk of having a baby with Down’s syndrome has been the combined first trimester screening test, sometimes known as the nuchal scan or the 11-1 4 week scan. Babies with Down’s syndrome will often have more fluid at the back of the their neck (an increased nuchal translucency measurement); a flatter facial profile with an absent or small nasal bone; abnormal flow of blood around their heart and through the liver; and a different amount of two placental hormones that can be measured in the mother’s blood. None of these markers will tell you that the baby definitely does or doesn’t have Down’s syndrome, but most affected babies will have one or more of these features and looking at all these markers will identify about 95% of affected babies as being at high-risk. Of course this means that 5% of affected babies will be given a low-risk result and may go undetected (a false negative result). Furthermore about 3% of normal babies will be given a high-risk result. The majority of parents with a high-risk result will choose to have an invasive test to make a definite diagnosis and most of these will subsequently find that the baby is unaffected (a false positive result).