Genetic testing is optional. It is not a required or necessary part of normal pregnancy care. Many people expecting a baby choose to do it, while others decide against it. This subject is confusing, and it can be hard to decide whether you want these tests.
Chromosomes are the genetic information that people get from their parents. They are like a set of instructions which tell the body how to develop and grow, while ensuring everything works properly. Most healthy people have 46 chromosomes inside every cell of their body. These 46 chromosomes are in 23 pairs. Each pair contains one chromosome from the mother and one from the father. This combination of chromosomes is one reason why many of us are like our mothers in some ways, and like our fathers in others.
If there is a mistake when an egg cell or sperm cell is made, a baby can receive one or more extra chromosomes, or have some missing. That baby is said to have a genetic abnormality. Some chromosome glitches have almost no effect at all, while others can be so serious that the baby dies, sometimes even before it is born.
Some genetic disorders that we can test for in pregnancy are familiar, like trisomy 21, which causes Down Syndrome. Others are rare. While certain disorders are serious or life-threatening, others are very mild. The disorders we most commonly screen for are trisomy 13, trisomy 18 and trisomy 21. All genetic screening tests look for these three genetic disorders. Some screening tests can check for other disorders too.
Doctors use two kinds of tests to look for diseases - screening tests and diagnostic tests.
Screening tests give us an idea of whether a person is at risk of having a disease or problem. They cannot tell us for certain whether the person actually has that problem. Diagnostic tests are more accurate, and can tell us for sure (or almost for sure) whether the person has a disease.
Why use a screening test if the diagnostic test is more accurate? Some diagnostic tests are difficult to do, are expensive, or have risks. In such cases, we do not want to use diagnostic tests for everyone – just for people who we really think might have the disease. The screening test can be an easier, less expensive or safer way to help us figure out who is at risk and needs more testing. The diagnostic test can then be used just for those high-risk people, instead of for everybody.
We only have a few diagnostic tests for genetic testing during pregnancy. They are difficult to do, expensive and carry a small risk of causing a miscarriage. For some people, it might make sense to skip the screening tests and do a diagnostic test right away. This includes people who have a genetic problem themselves or one that runs strongly in their family. They may also be at high risk of having a baby with a genetic disease for some other reason. However, for most healthy people who want genetic testing, a screening test is recommended first. A diagnostic test will be done only if the screening test shows a high risk of a problem.
Some genetic screening tests include an ultrasound, which can help your doctor or midwife figure out your due date. If you decide that you do not want genetic screening, or if you choose a test with no ultrasound (like cell-free DNA), you do not have to miss out on this part. You can have an ultrasound just to find out your due date, without doing genetic screening.
Examples of common genetic screening tests in pregnancy include nuchal translucency, first trimester screening, and cell-free DNA testing. Examples of diagnostic tests are chorionic villus sampling (CVS) and amniocentesis.
If you are pregnant and thinking about genetic tests, your first decision is whether you want to do genetic screening at all. Much depends on what you would do if the test shows a high risk of a serious chromosome problem. You have two things to consider.
First, if your screening test were to be abnormal, you must then decide whether you want a diagnostic test. Screening tests can have false-positive results, which means that many people with an abnormal (high risk) screening result actually have a normal baby. If your screening test shows a high risk of a problem, the only way to know for certain whether the screening test is correct is to do a diagnostic test. A diagnostic test gives a definite answer about whether there is a problem. Unfortunately, diagnostic tests can cause miscarriage or other complications. Due to these risks, many people do not want to have a diagnostic test. If you are sure that you would not want to have a diagnostic test, then you might decide not to do genetic screening in the first place. If the genetic screen is normal (low risk), it would be reassuring. However, an abnormal (high risk) result would cause you worry. Without the diagnostic test, you have no way to find out for certain whether there really is a problem. If you are not willing to take a diagnostic test afterward, it might be better not to do the screening test.
Next, you must consider is what you would do if you had a diagnostic test which showed that your baby really did have a serious chromosome abnormality. If you are sure that in that case you would want an abortion, then screening might be a good idea. Screening gives you information to help decide about diagnostic testing, which can help you to make an important decision. If abortion is not an option even if your baby has a serious abnormality, then the benefit of screening is less clear. Learning that your baby has a disorder might help you prepare for the challenge of raising a baby with a disability. It might also cause you a lot of worry. People who would not abort their pregnancy often choose not to take a screening test, as the answer would not change anything for them.
Of course, try not to panic if you have a screening test that does show something abnormal. Remember that screening tests are not always accurate. Before you make any big decisions, a diagnostic test must confirm there is actually a problem.
Many different genetic screening tests are available. Which ones you could have will depend on how far along you are in your pregnancy, what is available where you live, and whether you want to pay for the test. Here, we discuss only the three most common tests.
Nuchal translucency is a special ultrasound done between 11 and 14 weeks of pregnancy. It mostly looks at the thickness of the skin on the back of the baby’s neck. Babies with a thick layer of neck skin are more likely to have a genetic disorder. This test is done using just ultrasound, and is not very accurate. It has a high false-positive rate, meaning that most people who have a positive (high risk) test result actually turn out to have a completely normal baby. This can be stressful! If the nuchal translucency is abnormal, a diagnostic test is needed to find out for sure if the baby has a genetic problem. If you are trying to decide whether to have this test, think about whether you want to go through the stress and risk of diagnostic testing, as well as what a positive diagnostic test might mean for you.
The nuchal translucency test is covered by provincial health insurance, so you would not have to pay for it. One good thing about the nuchal translucency test is that, like all early ultrasounds, it can help your doctor or midwife figure out your due date.
Like nuchal translucency, the first trimester screen is also done between 11 and 14 weeks of pregnancy. It includes a nuchal translucency ultrasound, and also blood tests. Adding blood test results to the ultrasound makes the test much more accurate. Unfortunately, like most screening tests, it still has a high false-positive rate (though not as high as the nuchal translucency). Most people who have a positive (high risk) first trimester screen result actually have a completely normal baby. So, if the test is positive, it is still necessary to have a diagnostic test before making any important decisions. The first trimester screen is covered by government health insurance. Like nuchal translucency, a first trimester screen can also be a good way to confirm your due date.
Sometimes, people talk about a nuchal translucency test when what they really mean is a first trimester screen. If your doctor or midwife talks to you about having this test, ask them whether it includes blood testing, or just an ultrasound. If you have a choice, always ask for the one that includes blood tests. It is more accurate.
Cell-free DNA is just a blood test, with no ultrasound. It can be done any time after about 10 weeks of pregnancy. It is a fairly new test. Even without ultrasound, cell-free DNA is much more accurate than either the nuchal translucency or the first trimester screen. It is more likely to detect real problems, and also less likely to give a false-positive result. It tests for more genetic disorders than the other two screening tests. If you want, it can even give you extra information like the sex and blood type of your baby. As it does not include an ultrasound, it cannot help you figure out your due date. Don’t worry – if you choose this test, your care provider can send you for a separate ultrasound to find out your due date.
Cell-free DNA testing is expensive. Government health insurance does not pay for it, except in special cases in a few provinces. Sometimes private insurance such as work benefits will help pay for it, but most of the time people need to pay for it themselves. It usually costs between $500 to $800.
If you decide to do a cell-free DNA test, you have many choices to make about testing for disorders. Any genetic screening test can check for the main three genetic disorders, trisomy 13, trisomy 18 and trisomy 21. Cell-free DNA testing lets you test for many other genetic conditions besides these three. Your midwife or doctor can explain these to help you decide which ones you want to do. If you are still not sure, you might want to know what the experts say. The Society of Obstetricians and Gynaecologists of Canada is a group of specialists who give advice to doctors and midwives. For most people, they recommend only using use cell-free DNA to test for trisomy 13, 18 and 21. The test is most accurate for these disorders.
Even though cell-free DNA is a very accurate test, it is not perfect. It can still have false-positive results. If you do a cell-free DNA test and the result is abnormal or high risk, it always makes sense to consider doing a diagnostic test before making any decisions.
If you are trying to decide about genetic screening, all of this information may seem overwhelming. Talk to your doctor or midwife. They can explain what tests are available where you live, and which ones you can do at your current stage of pregnancy. They can also help you decide whether to do screening at all. Your partner or a trusted friend may be able to help you make this decision too. Remember that genetic screening is optional. Often people want to do a genetic screening test, but many people choose not to do any of this testing. It is your pregnancy and your choice.
The information presented here is meant for healthy people with normal pregnancies. If you or someone in your family has a genetic disorder, or if you are pregnant with twins or triplets, or have some other special situation, it may not be as helpful for you. Please talk to your doctor or midwife about what is best in your situation.