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  Prenatal Screening for Down Syndrome Download

Most pregnancies are normal but about 2% are complicated by congenital anomalies. Down syndrome is one of the anomalies that can be detected before birth. This leaflet is to help you understand Down syndrome, available tests offered by the Hospital Authority, and to help you decide whether you want the tests for Down syndrome or not.
What is Down syndrome? Most of us have 23 pairs of chromosomes in each cell. Individuals with Down syndrome have one extra chromosome 21 in their cells. Down syndrome occurs most often by chance and is seldom inherited from parents. Drugs, X-rays or other environmental exposure does not incur Down syndrome. Down syndrome is a condition with mental handicap and certain physical features different from other people. Some children with Down syndrome have abnormality such as heart defects. They have varying degrees of learning disabilities. With special care and education, some of them can integrate into normal schools and lead semi-independent lives. How common is Down syndrome? About 1 in 700 newborn babies have Down syndrome. The chance of having a baby with Down syndrome increases with mother's age. Table 1 shows the risk of having a live-born baby with Down syndrome at different maternal age. Table 1, If Age at expected dates of delivery is below 25, the risk having a live-born baby with Down syndrome is 1 in 1500. at 25, 1 in 1350 at 26, 1 in 1350 at 27,¡@1 in 1200 at 28,¡@1 in 1100 at 29,¡@1 in 1000 at 30,¡@1 in 910 at 31,¡@1 in 800 at 32,¡@1 in 680 at 33,¡@1 in 570 at 34,¡@1 in 470 at 35,¡@1 in 380 at 36,¡@1 in 310 at 37,¡@1 in 240 at 38,¡@1 in 190 at 39,¡@1 in 150 at 40,¡@1 in 110 at 41,¡@1 in 85 at 42,¡@1 in 62 at 43,¡@1 in 50 at 44,¡@1 in 35 at 45,¡@1 in 30 at 46,¡@1 in 20 at 47,¡@1 in 15 at 48,¡@1 in 11 at 49,¡@1 in 8 at 50,¡@1 in 6
Is there any advantage knowing if my baby has Down syndrome before birth? This would allow time for parents and doctors to discuss options to the best interest of the baby and the family. How can I tell whether my baby has Down syndrome before birth? A logical approach is to undergo a screening test to assess your risk or chance of having a baby with Down syndrome. The test does not harm you or your baby. It projects an estimated risk of your baby having Down syndrome, which is a more accurate estimate than that derived from your age alone. The Hospital Authority provides two screening alternatives: Screening test 1 Integrated Test (IT) - You will undergo an ultrasound examination at 11 to 13 weeks and 6 days of gestation followed by a blood test at 16 weeks to measure alpha-fetoprotein and human chorionic gonadotrophin levels. You will get the result within 2 weeks after the blood test. The test detects around 86% of Down syndrome pregnancies. This test is not suitable for you if you are pregnant for more that 14 weeks. Screening test 2 Blood Test only - If you are pregnant for more than 14 weeks, you will undergo the blood test at 16 to 19 weeks and 6 days for alpha-fetoprotein and human chorionic gonadotrophin. This test detects around 60% of Down syndrome pregnancies. For the above tests, about 5% of women will get a positive result, while about 95% will get a negative result.
Does a negative screening test result guarantee a normal baby? No, a negative screening test result indicates that the chance of your baby having Down syndrome is small. It does not guarantee a normal baby. If my screening test result is positive, how can I confirm if my baby has Down syndrome before birth? You need to undergo a diagnostic test in the form of amniocentesis. A diagnostic test will tell you accurately whether the baby has Down syndrome or not. This involves introducing a needle under ultrasound guidance into the uterus to draw amniotic fluid which contains cells from the pregnancy for chromosomal study. Amniocentesis is usually performed between 16 to 20 weeks. It is associated with a 0.5-1.0% risk of miscarriage. Can I undergo a diagnostic test without having the screening test first? The screening test is not 100% accurate. Some parents may not want to bear the risk, however small of missing diagnosis of Down syndrome in the baby. Therefore, having a direct diagnostic test without undergoing a screening test first is still an option for some women. However, this will be at the expense of increasing the chance of losing a normal baby after an unnecessary diagnostic test. If this is that what you want, the Hospital Authority will provide you with a list of private doctors or organizations for these tests.
What will happen if the fetus is confirmed to be abnormal? The doctor will explain to you and your spouse about the nature of the abnormality, its effect on the fetus and the risk in future pregnancies. You can discuss with the doctor about available support from the hospitals and other organizations and be better prepared for the birth of your baby. You may also discuss with your doctor about the option of pregnancy termination for fetuses with severe congenital anomalies, which is allowed by law in Hong Kong up to 24 weeks of pregnancy. Must I have the screening test? The screening test is voluntary. Not all parents want to find out if their baby has Down syndrome before birth because: After reading Table 1, you may be reassured that your chance of having an affected baby is low. You may choose not to terminate the pregnancy even if the baby has Down syndrome. Therefore, you may prefer not to have any test for Down syndrome before birth of the baby. You may not want to undergo amniocentesis even if the screening result is positive, because of the risk of miscarriage. Other points to note If you would like to know the screening result before 14 weeks of pregnancy, you should approach organizations outside the Hospital Authority for early screening, instead of joining the Hospital Authority fetal Down syndrome screening service. Join one screening program only. If you join more than one screening program, the results may be confusing and may cause unnecessary anxiety.
If you would like to know more about Down syndrome, you may contact the Hong Kong Down syndrome Association at 2718 7777 or browse through its web-site: www.hk-dsa.org.hk If you have further queries about the screening test, please contact your doctor.

 
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Last revision date: 01 September 2006