Genetic Screening and Testing During Pregnancy
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A detailed description of the genetic testing options available during your pregnancy.
While most babies are born healthy and without birth defects, approximately three to five percent of all babies are born with a birth defect. Some of these babies will have birth defects that are due to or associated with genetic disorders or syndromes. There are a variety of reasons that a pregnancy may be at increased risk for birth defects or genetic disorders. For example, if the mother or the father of the baby has a personal or family history of a genetic disorder or birth defect, that baby may be more likely to also have that birth defect or genetic disorder. If this is the case for you, please talk to your doctor or advanced practice nurse about scheduling a consultation with a genetic counselor to discuss possible risks to your pregnancy or future pregnancies. Many times, when a genetic disorder occurs in a pregnancy, it is due to a random genetic accident. Therefore, just because you and the father of your baby have healthy families and are healthy yourselves, this does not mean that your pregnancy may not be at risk for having a birth defect or genetic disorder. There are certain birth defects and genetic disorders that all pregnancies can be screened or tested for if a woman wants this information. All of these tests are optional and some women choose to do them while others do not. Some of the screening tests can be done prior to pregnancy (preconception), while others can be done only during pregnancy. The following sections will provide an overview of these optional tests, as well as how to think about these choices:
Birth defects commonly screened or tested for during pregnancy Down syndrome, trisomy 18, and open neural tube defects (like spina bifida) are the birth defects that all women are offered screening or testing for during pregnancy. Some tests will also provide information about trisomy 13 and other chromosome abnormalities. Down syndrome, trisomy 18, and trisomy 13 are chromosome abnormalities that are caused by a baby accidentally getting an extra chromosome (the chemicals in our cells that carry our DNA and genetic information) in the egg or the sperm. People with Down syndrome have mild to severe mental retardation and an increased risk for other issues such as heart defects. Babies with trisomy 18 or trisomy 13 are very severely affected with birth defects and mental retardation and the majority do not survive past one year of age. The chance of a baby being born with a chromosome abnormality like Down syndrome, trisomy 18, or trisomy 13 does increase gradually as a mother gets older; however, all babies are at risk for having a chromosome abnormality, even if a mother is very young. Open neural tube defects, like spina bifida, happen when the development of a baby’s spinal cord is interrupted and does not fully close into a tube. This is usually a randomly occurring birth defect, but if someone in your family or the baby’s father’s family has spina bifida, the risk to your children may be higher. It is extremely important to take folic acid with your prenatal vitamin (ideally before you even start trying to become pregnant), as this decreases the chance of your baby having spina bifida. The symptoms of open neural tube defects are extremely variable. In very severe cases, babies cannot survive, while in the mildest cases babies may require surgical repair but will have very few, if any, associated health problems. Many individuals with spina bifida will have partial paralysis or bladder issues, while others will have learning problems or mental retardation. Screening vs. diagnostic testing Screening tests and diagnostic tests are two different options for getting more information about the chance that your pregnancy has certain birth defects. Screening tests are done with ultrasounds, blood tests, or both. Diagnostic tests are considered “invasive” tests and involve inserting a needle into the area around a pregnancy. The benefits of the screening tests are that they involve no risk to you or the pregnancy for miscarriage and they provide additional information about the chance that your pregnancy has certain birth defects. The limitations of screening tests are that they will not give you a “yes or no” answer about whether or not your baby has a birth defect and they only provide information about a limited number of birth defects. Screening tests are designed to determine a more accurate risk (or chance) for birth defects, not to diagnose them. Most women who have screening tests use them to help decide if they want to proceed with further diagnostic testing. The benefits of the diagnostic tests are that they diagnose certain birth defects, giving you a “yes or no” answer, and that they look at more birth defects than screening. The risk of diagnostic tests is that, because they are invasive tests, they involve a relatively small risk of miscarriage. Screening tests for common birth defects in pregnancy The three most common screening tests offered during pregnancy are the sequential screen, the maternal serum quad screen, and the 20-week ultrasound. Sequential screen The sequential screen is a screening test that combines three pieces of information obtained during two stages of pregnancy to determine a pregnancy’s risk for Down syndrome, trisomy 18, and open neural tube defects (like spina bifida). The first part of the sequential screen is performed in the first trimester, between 10 to 13 weeks of pregnancy. At your sequential screen appointment, you will meet briefly with a genetic counselor then you will have an ultrasound. An ultrasound measurement of the fluid at the back of a baby’s neck (nuchal translucency) is taken. After the ultrasound you will have blood drawn. Information about your pregnancy is then combined with the information from the ultrasound and the blood work to determine a preliminary (or part I) result, which is available about one week after your appointment. This result will give you a specific risk for Down syndrome and trisomy 18 in the pregnancy. If these risks are higher than you are comfortable with, you will be immediately offered the option of proceeding to diagnostic testing to get a “yes or no” answer. If you do not elect diagnostic testing after the part I result, then you will return for a second blood draw in the second trimester, usually between 16 and 18 weeks of pregnancy. Information from this blood draw will be combined with the information obtained in the first trimester and about one week later you will get your final risk assessment. With the final result you will get specific risks for Down syndrome, trisomy 18, and open neural tube defects like spina bifida. Based on these results, you can decide if you want to have diagnostic testing. Maternal serum quad screen The maternal serum quad screen, or quad screen for short, is a screening test that looks at the levels of certain proteins made by a pregnancy that end up in a mother’s blood during the second trimester of pregnancy. By looking at information about your pregnancy and the level of these proteins, risks for Down syndrome, trisomy 18, and open neural tube defects (like spina bifida) in your pregnancy can be determined. Quad screening involves only a blood draw, which is done by your doctor or advanced practice nurse in the office between 15 and 21 weeks of pregnancy. Based on these results, you can decide if you want to have diagnostic testing or not. Quad screening is not as accurate as the sequential screen, but it is offered to women who did not have or were not able to have the first trimester part of the sequential screen. 20-week ultrasound The 20-week ultrasound is an ultrasound that is performed during the second trimester of your pregnancy, usually between 18 and 22 weeks of pregnancy. While most people think of this as the ultrasound during which you can find out the gender of your baby, it is actually performed to examine the baby in a safe way looking for the risk of many birth defects. This ultrasound will look for the presence of multiple structural birth defects, like heart defects, cleft lip, kidney problems, limb abnormalities, and differences in the way the brain is forming. While ultrasound can detect or diagnose a small number of structural abnormalities like cleft lip or club foot, it is also used as a screening test for some genetic disorders like Down syndrome. If certain differences are noticed in your baby’s ultrasound, a doctor or genetic counselor will discuss these findings with you, help you understand what the risk to your pregnancy may be based on these findings, and talk to you about your option for further testing with amniocentesis. Most babies with “abnormal” ultrasound findings will be healthy and will not have a genetic disorder. However, some babies with a “normal” ultrasound will end up having birth defects or genetic disorders that we cannot detect using ultrasound. |
Diagnostic tests for common birth defects in pregnancy
There are two common diagnostic tests performed during pregnancy:
Chorionic villus sampling (CVS) CVS is a diagnostic test that is performed during the first trimester, typically between 10 to 13 weeks of pregnancy. The main purpose of CVS is usually to determine whether or not a baby has a normal number of chromosomes (46). Therefore, CVS is diagnostic for birth defects like Down syndrome, trisomy 18, trisomy 13, and some other chromosome abnormalities. CVS can also be used to test for some other genetic conditions that your baby may be at risk for based on family history or carrier screening. CVS is generally performed in one of two ways, either by inserting a needle through a mother’s abdomen or through the cervix into a pregnancy’s placenta. A small piece of the placenta is then removed and sent to the laboratory for genetic testing. Results from a CVS are usually available about one to two weeks after the procedure. This risk of miscarriage after a CVS is approximately 1/250 to 1/300. Patients who have CVS performed will need a blood test in the second trimester to evaluate for risk of neural tube defects. Amniocentesis Amniocentesis is a diagnostic test that is performed during the second trimester, typically between 15 and 18 weeks of pregnancy, but can be performed at any time during a pregnancy. The main purpose of amniocentesis is usually to determine whether or not a baby has a normal number of chromosomes (46). Therefore, amniocentesis is diagnostic for birth defects like Down syndrome, trisomy 18, trisomy 13, and some other chromosome abnormalities. Amniocentesis can also be used to test for some other genetic conditions that your baby may be at risk for based on family history or carrier screening. It is also normal to use an amniocentesis sample to screen a pregnancy for open neural tube defects (like spina bifida). Amniocentesis is done by inserting a needle through a mother’s abdomen into the amniotic fluid that a baby is floating in inside the uterus. About three tablespoons of the amniotic fluid is then removed through the needle under ultrasound guidance. In that fluid are floating some of a baby’s cells that have naturally fallen off as a baby develops. These cells are then used by the laboratory for the genetic testing. The risk of miscarriage after an amniocentesis is 1/500 to 1/1000. If you plan on a having a CVS or an amniocentesis, it is an outpatient procedure and you should expect to be at the appointment for two to three hours. Prior to your CVS or amniocentesis, you will meet with a genetic counselor to review your family history and the details of the procedure. You will then be sent to ultrasound, where one of our OB-geneticists or maternal-fetal medicine doctors will perform the CVS or amniocentesis. After your CVS or amniocentesis, you are free to go home or to work; however, we recommend that you avoid strenuous activity for 24 hours. Carrier screening As you may remember from biology class, all of us have thousands of genes in our bodies. These genes are made up of DNA and they provide the instructions that our bodies use to grow, develop, and function. We all have two copies of every gene. For certain genetic disorders to occur, a person with that disorder must have two copies of the same gene that are not working correctly. As long as a person has at least one working copy of these types of genes, he or she is healthy and usually has no signs or symptoms of the genetic disorder. People who have one working copy of one of these genes and one non-working copy are called “carriers”. Carriers usually do not have a family history of the genetic disorder. It is thought that we all “carry” at least 8-10 of these non-working genes in our DNA. Usually, the fact that we have these non-working genes doesn’t matter. However, if we have children with someone who carries a non-working copy of the same gene, these children are at increased risk for having the genetic disorder. Carrier screening is offered to all people who are planning a pregnancy or who are early in their pregnancy. Most carrier screening is offered based on the ethnicities of you and the father of the baby. This is because people of certain ethnicities are more likely to be carriers of certain disorders. Some of the common genetic disorders that we offer carrier screening for are cystic fibrosis (CF), sickle cell anemia, Tay Sachs disease, and spinal muscular atrophy (SMA). Your doctor or advanced practice nurse will let you know which carrier screens are appropriate for you if you want them. Carrier screening is done through a blood test on a pregnant woman or the father of the baby. Carrier screening can also be done prior to pregnancy (preconception) and is encouraged for certain ethnicities, like individuals of Ashkenazi Jewish descent. The results of a carrier screen do not change from pregnancy to pregnancy and therefore do not usually need to be repeated. If both the mother and the father of a baby are carriers for the same genetic disorder, testing can be done prior to or during a pregnancy to determine if that particular pregnancy is affected (has both of the non-working copies). However, there are often no cures or treatments for these disorders. How to decide about screening and testing for birth defects during pregnancy There is no right answer about whether or not a woman should have screening or diagnostic testing for birth defects during pregnancy. All pregnant women are offered these tests, but it is a very personal decision whether or not to actually do them. Women use all sorts of different ways of making decisions. Sometimes these decisions are made based on a woman’s personal experience with family members or friends with birth defects or disabilities. Sometimes these decisions are made based on financial considerations or support resources available to a family. Some women are information-gatherers who find it easier to cope with pregnancy if they have all possible information available, while other women find it easier to cope with pregnancy when they focus on the most likely positive outcomes instead of relatively small risks. Some women make these decisions based on whether or not the results would change their plans for the pregnancy, while other women make these decisions based on planning to continue a pregnancy no matter what, but wanting time to prepare for having a baby with a birth defect. Again, there is no right answer to these questions. If you feel that additional information would be helpful to you, you should choose between the screening and diagnostic options. If you feel that this is not information you want about your pregnancy, you can choose not to have any of these tests. A lot more information about each of these screening and testing options is available for women who want it. If you have questions about your options or want help making a decision, please speak to your doctor, your advanced practice nurse or a genetic counselor. |
