Prenatal screening and testing

Transcription

1 Prenatal screening and testing

2 Pregnancy is often a joyful time and most births result in healthy babies; however, 3 to 5 percent of all babies born have a birth defect or genetic condition. Many of these are isolated birth defects that can often be repaired after the birth of the child. There are tests that can be performed during pregnancy that can help find some of the more common problems seen in newborns. This brochure provides information about testing options and the differences between them. Chromosomal Conditions Down syndrome (trisomy 21) Down syndrome is a genetic condition in which there is an extra copy of the 21st chromosome. Instead of having the usual two copies of the chromosome, there are three copies. This extra chromosome will cause mental retardation and can often cause heart defects and other birth defects. Most people with Down syndrome will learn to walk, talk and do basic skills by themselves, but as they get older, they will need help in school. As young adults, they may be able to get a job and live on their own, but they will need some help from other adults. There is a wide range of learning abilities seen in people with Down syndrome. It is not possible to tell before a baby is born the exact learning abilities a person with Down syndrome will have. Other Conditions Open neural tube defects (spina bifida) Spina bifida is an opening along the spine which damages the nerves. This damage leads to hydrocephalus (water on the brain), difficulties with walking and often trouble with bowel and bladder control. The walking difficulties can range from needing crutches to a wheelchair. Although surgery to fix the opening is done soon after birth, the damage to the nerves takes place during pregnancy and cannot be corrected. There are a few hospitals in the country that offer repair of the opening before the developing baby is delivered. It is thought that this fetal surgery can help improve the outcome for these children. All women have a chance for having a child with spina bifida it happens in one to two of every 1,000 births. Screening and Diagnostic Tests Screening tests provide information about the chances of having a baby with certain conditions. These tests will not provide a yes or no answer as to whether a baby definitely has a health concern. In order to be certain, more diagnostic testing usually needs to be done. Screening tests are not harmful to a mother or pregnancy and are usually done through blood tests and ultrasound examinations. Diagnostic tests tell with more certainty if there is a health condition in a baby. If a screening test comes back with a higher chance for a specific condition, chorionic villus sampling (CVS) or amniocentesis will be offered. Unlike screening tests, diagnostic tests carry some risk. A diagnostic test may also be offered if there are other risk factors, such as a mother who is 35 years or older, or if there is a family history of a genetic condition. There are both screening and diagnostic tests that are offered in the first and second trimesters of pregnancy. There is no right or wrong decision about whether to have screening or diagnostic testing. Before screening and testing, think about: How worried are you about the conditions these tests can detect? How would you use the information from these tests? - To plan for the birth of a baby with special needs? - To learn more about how to care for a baby with special needs? - To consider termination of pregnancy? - To consider adoption? Would it worry you more or less to know that your baby has one of these health conditions before the baby is born? Understanding that diagnostic tests carry some risk, are you willing to take the risk to get more clear answers? If you would never want to do an invasive diagnostic test, should you have a screening test if the follow-up to a positive screening result is a diagnostic test? Trisomy 18/trisomy 13 These are chromosomal conditions that are less common than Down syndrome and are due to having an extra copy of the 18th or 13th chromosome. Children with either of these conditions will have many different birth defects and severe mental retardation. Only a few children with either condition will live past their first birthday. Who is at risk? Anyone can have a baby with Down syndrome or one of these chromosomal conditions. These conditions happen by chance and do not run in families. Chances increase with a mother s age. For example, the chance of delivering a baby with Down syndrome ranges from about one in 1,300 at age 20 to about one in 80 at age 40. It is important to know that there are no cures for any of the above conditions.

3 Screening Test First Trimester Screening Test Second Trimester The First Check screening test is done between nine and 14 weeks of pregnancy. It can help find those pregnancies which may be at an increased risk for chromosomal conditions such as Down syndrome (trisomy 21), trisomy 18 and some cases of trisomy 13. The test is done by drawing a small amount of the mother s blood and using an ultrasound to take a special measurement of a fluid-filled space at the back of the baby s neck (the nuchal translucency). The blood test measures two chemicals (PAPP-A and beta hcg) that are normally made during pregnancy. The Quadruple Check is a blood screening test that can help find pregnancies which may be at higher risk for chromosomal conditions, such as Down syndrome (trisomy 21) or trisomy 18, or other birth defects like spina bifida. The Quadruple Check measures four chemicals in a pregnant woman s blood: alpha-fetoprotein (AFP), unconjugated estriol (UE3), human chorionic gonadotropin (hcg) and dimeric inhibin A (DIA). These chemicals are produced by the developing fetus and placenta. It is normal for these chemicals to be found in the blood during pregnancy. When is the First Check performed? Blood can be drawn anytime between nine and 13-6/7 weeks and the ultrasound should be performed between 11-1/2 weeks and 13-6/7 weeks. If blood is drawn at least a week before the ultrasound, First Check results can be given by a genetic counselor or physician right after the ultrasound. This is called an Instant First Check. When is the Quadruple Check performed? Blood should be drawn between 15 and 20 weeks of pregnancy, although the best time to have this test is between 16 and 18 weeks. For earlier pregnancies, consider having a First Check screening for the same chromosome conditions between nine and 14 weeks of pregnancy. Does the First Check find all children with these chromosome conditions? No. The First Check can find approximately 80 to 90 percent of babies that have Down syndrome and 97 percent of babies that have trisomy 18. Can the First Check find any other health problems? If the nuchal translucency measurement is increased, concern is raised for other health conditions, such as heart defects. The First Check does not check for neural tube defects, such as spina bifida or an open spine. Neural tube defects can be looked for during an ultrasound in the second trimester. Does the Quadruple Check find all children with these conditions? No. The Quadruple Check can find approximately 75 to 80 percent of babies that have Down syndrome and 60 to 80 percent of babies that have trisomy 18. The Quadruple Check can also find 80 to 85 percent of open spina bifida cases. What is the difference between the First Check and screening that is done in the second trimester? Both of these screening tests look for pregnancies which may be at a higher risk for certain chromosomal conditions. The First Check is done earlier in pregnancy and finds more affected babies than second trimester screening. The First Check does not, however, look for neural tube defects while second trimester screening can detect this type of birth defect. Contingency screening first and second trimester combined Contingency screening combines both the First Check and the Quadruple Check to give risks for having a pregnancy with Down syndrome, trisomy 18 and neural tube defects. With this screening, a result is provided after the first trimester testing. Based on this result, women with low risk have no further testing, while others with higher risk have the second part of the screening or are offered a diagnostic test. For example: If the First Check gives a low risk for these conditions and the Quadruple Check would not change that risk, no further testing may be done. Other times, the First Check results show some risk and it is recommended to have second trimester screening to give a better idea of the chances for these conditions. Sometimes, the risk from the First Check is at a high level where diagnostic testing is offered in the first trimester because the Quadruple Check won t lower the risk very much. A diagnostic test can be requested after the first or second trimester. Does the contingency screen find all children with these conditions? No. If both parts of the contingency screening are performed, approximately 94 percent of babies that have Down syndrome and 95 percent of babies that have trisomy 18 or trisomy 13 can be detected. The contingency screen can also find 80 to 85 percent of open spina bifida cases. Second trimester ultrasound This screening is performed around weeks of pregnancy. Half of all babies with Down syndrome and more than 90 percent of babies with trisomies 13 or 18 will have a finding that can be seen on ultrasound. Ultrasound is mainly used to screen for birth defects or clues that a baby may have Down syndrome. Birth defects are seen in 2 to 3 percent of all pregnancies and can happen to anyone even if there are no birth defects in a family or high-risk medications used during pregnancy. An abnormal screening result does not mean that there is definitely a problem in a pregnancy. It means that further testing will be offered. A normal screening result will not guarantee that a baby will be normal. It will, however give women and health care providers very important information that pregnancy is proceeding normally and chances for a healthy baby are excellent.

4 What is Diagnostic Testing? What is Genetic Counseling? Screening Test First or Second Trimester Noninvasive prenatal testing (NIPT) A new screening test has become available that can detect chromosome conditions like Down syndrome, trisomy 18 and trisomy 13 by a blood test done on the mother. We know that when women are pregnant, some of the baby s genetic material gets into the mother s blood stream and the mother s blood can be tested for chromosome conditions. One benefit of this test is that it has no risk to the pregnancy; however, the test is new and there is not much experience with how well the test works in all situations. When is NIPT performed? NIPT is a blood test on the mother that can be done between 10 and 31 weeks of pregnancy. Does NIPT find all children with these conditions? No. This screening can find over 99 percent of babies with Down syndrome, 97 percent of babies with trisomy 18 and about 80 percent of babies with trisomy 13. In most cases, when a baby is healthy the test will be negative, but on a rare occasion the NIPT could be positive or abnormal, with a healthy baby. For this reason, everyone with a positive test needs additional diagnostic testing such as CVS and amniocentesis (see following section) to confirm the results. Who can have NIPT? This test was originally created for women who will be 35 at delivery, women who have a positive result from other screening tests, or women who have had a previous pregnancy with a chromosome condition. Therefore, we have the most information about how this test works for these women. The test is now available for all women; however, to determine if it is the most appropriate test for you, speak to your health care provider or a genetics professional. Chorionic villus sampling first trimester Prenatal diagnosis checks a baby s chromosomes during pregnancy. One way is called chorionic villus sampling (CVS), which is done between 10 and 13 weeks of pregnancy. CVS usually uses a thin catheter (tube) inserted through the cervix to take a small piece of the placenta. The developing placenta is made up of the same cells as the developing baby and is tested to make sure that there are 46 chromosomes in the baby s cells. Amniocentesis second trimester Amniocentesis is done anytime after 16 weeks of pregnancy. Amniocentesis (or amnio ) is a procedure in which amniotic fluid is withdrawn from the uterus through a thin needle. Amniotic fluid contains alphafetoprotein (AFP) and cells shed from the baby s body. The amniotic fluid sample will be used to help rule out birth defects such as spina bifida and make sure that there are 46 chromosomes in the baby s cells. These tests (CVS and amnio) can identify 99 percent of babies with chromosomal conditions. Both of these tests have some risk for complications which may lead to miscarriage. Before considering prenatal diagnosis, speak to a health care provider and a genetic counselor to be informed of the risks and benefits of these tests. The goals of genetic counseling are to help understand screening test results and/or guide decisions about diagnostic testing. A genetic counselor will also ask about a family s health history to find out if a baby could be at risk for other conditions that are passed down in a family that the screening or diagnostic testing may not find. A genetic counselor can also talk about exposures during the pregnancy that may or may not be harmful to a developing baby. For more information, ask your doctor to refer you to your local genetics center or call the Center for Human Genetics at

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