Important: This is intended as general information only. It may not apply to your specific situation. It is not a substitute for medical advice. If you have a question or concern, please raise it with me at your next appointment or contact me sooner if it's urgent.
Reproductive Carrier Screening
At your first pregnancy visit you will be offered a test to see if you are a 'carrier' of three genetic diseases. This test is optional – you do not have to have it done if you do not wish to. Carriers are individuals who are healthy but have an abnormal gene which can be passed on to their children. Reproductive Carrier Screening (RCS) includes testing for three genetic diseases which are relatively common in out population - cystic fibrosis (CF), spinal muscle atrophy (SMA) and fragile X syndrome. CF and SMA are what are known as recessive diseases. For an individual to have these diseases, they must have two copies of an abnormal gene. We have two copies of all of the genes in our bodies - one from our mother, one from our father. When we make a sperm or an egg, we put one of our two genes (from each pair) into that sperm or egg. People who have one abnormal CF or SMA gene have no signs from the outside but can pass on that gene to their children. This becomes a problem if that child also inherits an abnormal gene from the other parent. Fragile X Syndrome works in a slightly different way - it is carried on the mother's side of the family but (usually) only affects boys.
Cystic fibrosis is one of the most common genetic diseases in our population. It is a severe disease that mostly affects the lungs, causing a build up of thickened mucous. This mucous can block the airways and trap bacteria causing a chronic cough and recurrent infections. Over time, these blockages and infections damage the lungs resulting in difficulty breathing. It also affects the pancreas which can make it difficult for sufferers to digest food. As a result, people with CF may suffer from malnutrition and poor growth. Treatment has improved over the years and many people with the disease lead full and productive lives. Nonetheless, it remains a serious disease requiring treatment on a daily basis and some people with the disease may go on to require a lung transplant. The average life expectancy for someone with CF is reduced at 35-40 years.
In order to have CF, a person needs to have two copies of an abnormal gene – they inherit one from each parent. About 1 in 25 people in the Caucasian population have one copy of the abnormal gene and are known as ‘carriers’. Carriers usually have no symptoms and are unaware that they have abnormal gene. If both parents are carriers, there is a possibility that their baby could inherit two copies of the abnormal gene and end up with the disease. However, not all children born to two carriers will have cystic fibrosis as each parent gives one gene only (either their normal or abnormal gene) and there is only a problem for the baby if he or she ends up with two copies of the abnormal gene. In the situation where both parents are carriers, there is a 1 in 4 chance that their child will have CF. While people from other races can carry the abnormal gene, it is much less common than in Caucasians.
We start by testing one parent (usually the mother) and if she is not carrier, their is no need to test the father. If the mother is a carrier, we then test test the father. If both parents carry the gene, we can then consider testing the baby.
Spinal Muscle Atrophy
Spinal muscle atrophy is genetic disease that causes progressive muscle weakness. This eventually affects the muscles required for breathing and, as a consequences, most children with SMA pass away before the age of two. It is less common than CF, with about 1 in 40 people carrying the gene. Similar to CF, it is only a problem if both parents carry the abnormal gene. When both parents are carriers, there is a 1 in 4 chance that their child will have SMA.
As for CF, we test the mother first and only test the father if the mother is found to be carrier.
Fragile X Syndrome
Fragile X syndrome is caused by an abnormality on the X chromosome. Women have two X chromosomes (XX) while men have one X and one Y (XY). For this reason, the disease only (generally) affects boys - women who have one abnormal X chromosome are not affected because their other (normal) X chromosome functions normally. Therefore, we test for Fragile X by testing the mother's DNA only - testing of the father is not required.
What happens if the test is abnormal?
If we find that both you and your partner carry the CF or SMA genes, we know that there is a 1 in 4 chance that your baby will have CF. If you wish, you can then go on to have genetic testing of your unborn baby to see if he or she will be affected. This is usually done by amniocentesis. If you carry the genetic abnormality that causes Fragile X syndrome, we can also perform an amniocentesis to test the baby.'
Who should have the test?
Not everybody wants to know whether they carry genetic diseases - this test is optional. Unfortunately, none of these genetic diseases can be cured - although hopefully this will change in the future!
You are offered this test so that, if you found out you were going to have a baby with CF, SMA of Fragile X, you would have the option of terminating the pregnancy. Of course, this is not a decision that you need to make before having the test and, at each step of the process, I would discuss all the options with you in detail. However, if you are certain that you would not consider a termination of pregnancy if you found out you were having a baby with these conditions, it’s probably best not to have the test in the first place – it’s just one more thing to worry about and one more thing to pay for! There is no right or wrong decision with this test. It’s a very personal decision and it’s completely up to you whether you have testing or not.
This test in only required once - we are testing your DNA which will not change. Therefore, if you are tested and found not to be a carrier in your first pregnancy, you do not require the test in future pregnancies.
It may be worth considering this test before you plan to become pregnant so I usually discuss it if I see patients for preconception counselling. If both partners carry a genetic disease, it may be possible to have IVF with a process known as Preimplantation Genetic Diagnosis (PGD). PGD involves creating embryos in the lab, testing the embryos to determine whether or not they are affected and only transferring embryos that are normal or carriers (i.e. not using embryos that have the disease). PGD is done through and IVF clinic after extensive genetic counselling. It is obviously a fairly complicated (not to mention expensive) process and not every body wishes to go down this pathway.
What other genetic tests are available?
If you or your partner have a family history of any genetic disease, there may be specific testing available. This is usually done in conjunction with a clinical geneticist. I'll ask you about your family history at our first visit.
Their are some other tests available that test for an extended range of recessive genetic diseases. These are performed overseas (in the US) and involve sending a sample overseas. Theses tests are more expensive that RCS and most of the conditions they test for are very uncommon. For most people with no family history, they are probably unnecessary but we will discuss them in more detail at your first visit.