Last Updated on Sunday, 01 August 2010 23:21 Written by Sandi Graham-McWade Friday, 30 July 2010 23:32
Different Types of Down syndrome & Prenatal Testing
The following is an article written by Stephanie Mamayson for the Canadian Down syndrome Society. This article describes the different types of Down syndrome.
Types of Down syndrome
There are three types of chromosomal patterns that result in Down syndrome. They are: trisomy 21(aka Nondisjunction Down syndrome - added note by Sandi Graham-McWade), translocation and mosaicism. A blood test is required for chromosome analysis and the type of pattern would be determined at that time.
Ninety-five per cent of people with Down syndrome have trisomy 21, which means there is an extra #21 chromosome in each cell.
Two to three per cent of people with Down syndrome have a translocation pattern. In translocation, during cell division, a part of the number 21 chromosome breaks off and attaches itself to another chromosome, usually the number 14 chromosome. About two-thirds of these translocations occur spontaneously while the rest are inherited from a parent. If the chromosome test shows a translocation, it is recommended that the parents have their own chromosomes analysed as the chance of having other children with Down syndrome may increase significantly.
Approximately two per cent of people with Down syndrome have mosaicism. In this case, a faulty cell division occurs in one of the early cell divisions after conception, resulting in some cells having 46 chromosomes and some having 47. The percentage of cells with 47 chromosomes instead of 46 varies from person to person.
Personal Perspective - written by Sandi
With: Types of Prenatal Testing
First a note: What is the difference between Prenatal Screening and Diagnostic Testing? The short answer is, a screening test tells you the CHANCE you would have in having a baby with a chromosomal/genetic abnormality. The Diagnostic Testing is what TELLS you for sure.
When I first found out that our son Hunter had Nondisjunction Down syndrome (the random genetically occurring Down syndrome), I was 18 weeks pregnant. I had been offered and subsequently given an amniocentesis, by personal choice. Originally I had no reason to want the amniocentesis, there were no increased risks other than my original maternal risk assessment (odds for Down syndrome) which was 1:385 ratio chance for the age of 35. I knew years ago, amniocenteses were only offered to women over the age of 35, because risks for genetic abnormalities were known to increase when the mother's age became something the medical world calls "Advanced Maternal Age" or being pregnant over 35. Most countries still run under this same protocol, but in the last few years, Canadian Women were being offered the choice of having an amniocentesis (despite their age) as another method of prenatal testing. A few of the prenatal tests that I had already done before deciding to have the amniocentesis were the Nuchal Translucency Scan (or NT scan), the Integrated Prenatal Screen (or IPS), and ultrasounds (sonograms).
To explain these tests I have put together the following:
Amniocentesis: An amniocentesis is an invasive type of prenatal test, which involves the use of a long catheter needle that is used to pierce the abdominal area through to the uterus to collect a sample of amniotic fluid that surrounds the fetus. This test is usually done at around 16 weeks in the pregnancy. A high level ultrasound is used to help guide the doctor find a suitable pocket of fluid where a small sample of amniotic fluid will be taken. The test is considered invasive since it involves obtaining fluids from within the body using a needle. There is a small risk of loss (miscarriage) associated with this test: 0.25% - 0.5% in addition to the risk of natural miscarriage percentage. I personally found this procedure very easy and not even the slightest bit uncomfortable. The literature about this test suggests that most women find the procedure easy and quite bearable, with little discomfort, however each woman is different. The results for the amniocentesis are 99% accurate, and 100% when you remove the possibility that paperwork or lab mix-ups happen and something even rarer still - the possibility of testing the mother's cells rather than the fetus's amniotic fluid occurr. The total of this occurrence happen is less than 1% of the time. And to rule out the possibility of testing the mother's cells, it is important to note that if you discover that your baby is male (X,Y chromosome) this would eliminate the possibility that the mother's cells were tested, since the mother is female (X,X chromosome). The testing of the chromosomes will yield something called a Karyotype. This is a chromosomal mapping that tells a geneticist if your baby has normally arranged chromosomes. It will also tell the geneticist if you are having a boy or a girl, since the sex chromosomes are included in the karyotype. It is also important for me to explain that only the most common chromosomal abnormalities are studied or looked for. Also, occasionally some amniocentesis also have the Alpha-Fetoprotein (AFP) measured to assist in detecting open neural tube defects. If the amniocentesis comes back "normal", it does not preclude the baby from having another (perhaps less common, rare or even undiscovered) genetic abnormality. You must ensure when you are having the amniocentesis done which genetic/chromosomal abnormalities are being tested for. The time frame for receiving these results differ with areas, but commonly you can expect your results to be given to you within three to four weeks, unless there is some abnormality discovered. If something is found, often the family will be notified sooner in order to arrange genetic counselling. (Genetic counsellors/doctors explain the results and discuss with you the options available to you going forward from there.)
Nuchal Translucency Scan: The NT scan is a high level special ultrasound performed at/between the weeks of 11 - 14 weeks gestation. It is important to have this test during that time frame because either earlier or later than those dates will yield unreliable results. Too early and it would be nearly impossible to detect, and too late the Nuchal area begins to diminish and the results would be inaccurate. There are many factors that go into the calculation of the measurement, some of which are gestational age, and Crown Rump Length of the fetus. Some additional calculations for adjustments per technician are also included because each technician is different when it comes to accurate measurements. The area which is measured is located beneath the head where the back of the neck is. There is a fluid and space that is measured in that area. As a result of the fluid, the fetus's nuchal fold increases in thickness (which is what is measured) and increases until around the 15th week of gestation which at that point in time the fluid disappears. The technicians record that measurement and it is then compared against the average (IMPORTANT NOTE: Averages from different areas and different countries are NOT the same.). The measurement which is applied to a formula is then compared to the average (your local average) and tells the doctors if there is a reason to suspect primarily some genetic or chromosomal abnormalities and a risk factor can be made. If the resulting measurement is greater than the average, you may be assessed as having an increased risk for the genetic or chromosomal abnormalities - such as Trisomy 21 (Down syndrome), Spina Bifida, Trisomy 13 or Trisomy 18. Having a positive screen or an increased measurement (usually greater then the average indicated for the area in which you live) doesn't mean that your baby will have one of these genetic concerns. It means that you would be recommended for further prenatal screening or testing. Often times this NT scan along with other prenatal tests can give a more realistic look at whether it is statistically more likely that you might have a baby with one of these abnormalities. But, having an increased measurement doesn't always result in these. It merely means that the risk for these is increased. By how much is what counts. It is important for me to note - that Hunter, (our son) had a perfectly normal NT scan, completed at 12 weeks. The measurement was 2.3mm and the general average is 2.5mm. For Canada, my genetics clinic advised me that the country's average is 3.5mm, and anything less than 3.5mm is considered normal. I have seen variances from 2.0 to 4.0 as normal median measurements in my research. So, before you can really understand if your measurement is normal, or there is a reason for concern, you must know what your range is (for average) is, based upon the guidelines used by your location. Only your genetics counsellor or doctor can tell you this information since it differs country to country, and even state to state. This is really important that I stress this because often I have many mom's asking other people through website forums or other friends or family members who have had this testing done, and when they compare their result to what their consulting person's was (along with that person's given average) it might make them believe that something is wrong, when in fact there isn't anything wrong. It may also lead the mom who is questioning and comparing her baby's measurement to believe that their test was wrong, faulty or not being done properly. Worse yet, it may lead that mother to believe that the next step in genetic testing that should be taken wasn't being given or offered as recommendation from their medical professional.
To sum up NT scans: Testing can show normal results (as mine did) yet in the end it can turn out that you have a baby with Down syndrome (or other chromosomal abnormality) or, it is also possible to have a positive scan where the results are elevated or above the given average and turn out that you have a baby without a chromosomal abnormality. So, what do we take away from this testing then? That is only provides doctors with a guide as to whether or not it is more likely that you may or may not have a baby with a genetic/chromosomal abnormality such as Down syndrome.
Integrated Prenatal Screening (IPS): The integrated prenatal screen or IPS is a collection of tests performed during a specific period of time to determine the chances of a problem (only certain abnormalities are detected with these tests). There are other tests that are offered before and after the IPS which are nearly standard screening tests for all pregnancies. In the first trimester, the First Trimester Screening (FTS) is conducted using a blood test at 11-14 weeks, the NT scan (high level ultrasound) at 11-14 weeks, with results available at 12-15 weeks. In this early testing, if results come back indicating the possibility of an abnormality discovered, a Diagnostic test will be offered. (At this early stage a CVS or Chorionic Villus Sampling test is the only Diagnostic test that can be offered at 11 weeks.) If no detection is made, the IPS screening will be continued/offered in the second trimester which involves all of the FTS, but also has a second blood test at 15-20 weeks. The results will be available between 16-20 weeks. If there is a detection made at this time, either a CVS as above or Amniocentesis to occur at 16 weeks will be offered as the Diagnostic test. The results of the Amniocentesis would be available from 17-24 weeks (depending on when it was performed). If the detection from the screening was negative, the next step in screening would more to the third trimester for the Serum Integrated Prenatal Screening or SIPS. These screening tests (FTS, IPS and SIPS) are available if your first visit to the doctor is before 14 weeks you can opt to have Triple Screening or Quadruple Screening (Quad screen is more accurate and often replaces the Triple screen). If your first visit is after 14 weeks Quad screening.) In this phase of the screening, all the blood work is the same but the testing is a two step process and this test (and the IPS) are more accurate then the First Trimester Screen, however the results are available later in the pregnancy. If there is a detection made at this stage, an Amniocentesis is usually recommended but a CVS can also be done. The availability is the same as above, 15-22 weeks, with the same for the results, 17-24 weeks. It is important to point out that using screening, there is the chance of having what is called "False Positives" (which is where the test will show an increased chance due to the results of the blood work or scan and yet the baby is perfectly fine). The detection rates for finding Down syndrome is about 80-85% in the first trimester and 80-90% in the second and third. However the percentage for false positives to occur can be anywhere from 2-9% but I was told and also researched that it could be up to 15%.
To sum up Screening using FTS, IPS or SIPS: Ultimately, having these tests gives you the ability to have screening done without having to do an invasive test first. The results of the screening give you good indications as to whether you should move onto more screening or have a diagnostic test to confirm the results. Choosing to do screening alone obviously keeps you from having to have any invasive testing done, but there is no way to definitively confirm a diagnosis. With screening alone (and no diagnostic test such as CVS or Amniocentesis) everything is only a possibility or speculation. With the Diagnostic testing, the benefits are that you have a definitive test and result, but the cons are that it is invasive and with invasiveness comes risk. Each person is different when it comes to how and what tests are chosen and performed. Every one has different reasons for wanting or not wanting to do screening/testing. Some people know that testing gives you the ability to have choices and subsequently the ability to make decisions based on those results. Other's know that regardless of the outcome they wouldn't change the way they handle their pregnancy so putting additional risk into the equation doesn't make sense. And even more reasons are because some people (like myself) are need to know type people, and if offered the chance to learn and research if something isn't typical, then ultimately the time that the results yield/give us, ensures that we can prepare for whatever the situation may be/bring.
What should I choose to do?
When it comes to screening, testing and results, the important thing to note is that the choice is a personal one, and there is no right or wrong. You might make this choice alone, with your spouse or partner or with your family members and even a friend(s). Sometimes religion or beliefs provide a clear path of choice, other times it isn't always so clear. But no matter what the decision, it is yours to make and no one should make that choice for you, or criticize you on whatever you choose. Sometimes the benefits outweigh the risks or maybe the risks outweigh the benefits. All reasons are valid, no matter how trivial it/they may seem. The question after testing is the one that becomes more important once you have the results. If you have learned that your baby has Down syndrome, you might want to read an article I wrote called, "Down syndrome - The Diagnosis" BEFORE you decide what to do next. I guarantee, having an informed decision is always better then choosing to do something, which without the benefit of current research or information will have you asking yourself down the road if you made the right decision. You can find this article "Down syndrome - The Diagnosis" on the Down syndrome menu or click here to get to it.
|< Prev||Next >|