Down Syndrome
Examinations and Tests
Testing before becoming pregnant
You may want to consider genetic testing for Down syndrome if you are planning to become pregnant and you have a family history of the condition. Genetic testing can confirm whether you or your partner carry the translocation chromosome, which increases your risk for having a child with the translocation type of Down syndrome. But this kind of testing does not detect whether your child is more likely to have the trisomy 21 type, which accounts for most Down syndrome cases.
Genetic counselling can help you understand why genetic testing is done, what the results mean, and how the results may affect you and your family.
Screening during pregnancy
Screening does not diagnose Down syndrome but rather provides information about the likelihood that your fetus will have the condition. Screening tests include:
- Fetal ultrasound. This test can detect signs of Down syndrome in the first trimester. A fetal ultrasound image can show a greater-than-normal amount of fluid at the back of a fetus's neck. The skin fold thickness measurement estimates the distance between the surface of the skin and the neck bones (nuchal fold). This test is not yet widely available, because only a specially trained provider can do it.
- Maternal serum triple or quadruple screen test. The triple test (triple screen) is done in the second trimester. It measures the amounts of the following 3 substances in your blood:
- Alpha-fetoprotein (AFP)
- Human chorionic gonadotropin (hCG)
- Estriol (uE3)
The test is called a quadruple screen (quad screen) when a test for inhibin A is added. Inhibin A is a protein that is produced by the fetus and the placenta. The quadruple test can detect most Down syndrome cases, and it may be less likely than the triple screen to deliver false-positive results.6 The triple and quad tests can only be done in the second trimester.
Other screening options that may be available in the first trimester include:
- Combination screening.
- Integrated screening.
A combination screening uses both a fetal ultrasound and a blood test to measure levels of human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein-A (PAPP-A). It has shown to be as exact as the quad screen for detecting Down syndrome.7 One advantage of this type of screening is that it can be done earlier in your pregnancy than some other tests.
Integrated screening uses both first and second trimester tests to predict the risk of having a child with Down syndrome. It is one of the most accurate tests that may be available.7 The results are reported only after both sets of tests are finished. This can be cause for concern for women who would like to know test results early on.
Screening tests for Down syndrome and other conditions require your consent. Some women find that knowing their chances of having a child with Down syndrome helps them to prepare for the possibility. Others are more comfortable going through their pregnancy without having this information.
Also, the type of screening tests given, alone or in combination, depends on the mother's goals and preferences. In Canada, the types of tests given also depends on which ones are offered. Triple screen is available in most places. For example, when a mother is most concerned with screening accuracy and does not want to have amniocentesis, her doctor may recommend a combination of fetal ultrasound and blood test during the first trimester plus a triple or quad screen during the second trimester.
Diagnosis during pregnancy
A chromosomal analysis, called a karyotype test, can diagnose Down syndrome during pregnancy. A sample is taken directly from the fetus or placenta using one of the following techniques:
- Amniocentesis. This usually is done at 15 to 18 weeks or later in your pregnancy. The liquid (amniotic fluid) that surrounds your fetus contains cells that have been shed by your developing baby. These cells can be checked for the number and size of chromosomes to see if there are any problems that put the baby at risk for certain cnoditions, such as Down syndrome. The test is done by inserting a needle through your abdomen into your uterus. Approximately 30 mL (2 Tbsp) of the amniotic fluid is collected and examined. Amniocentesis has a 1 in 370 risk of causing miscarriage.8 It is 99.8% accurate in detecting Down syndrome. Results of amniocentesis usually take about 2 weeks.
- Chorionic villus sampling (CVS). This test is done earlier than amniocentesis, ideally between weeks 10 and 12 of your pregnancy. Chorionic villi
are tiny finger-shaped growths found in the placenta. The genetic material in chorionic villus cells is identical to the genetic material in fetal cells. A biopsy of these cells can provide doctors with genetic information about your fetus. Doctors can then let you know whether your fetus is likely to have certain conditions, such as Down syndrome, that can develop from chromosomes that are not normal. The chorionic villus sample is usually collected through the mother's vagina. The doctor inserts a speculum to gently spread apart the vaginal walls, and a catheter is guided through the cervix to the placenta using ultrasound. When the catheter is correctly placed, a sample of chorionic villus cells is collected. Less often, the sample of chorionic villus cells is collected by inserting a needle through the abdomen into the woman's uterus. CVS has a 1 in 360 risk of causing a miscarriage.8 Results of CVS are usually available within several days.
Because there are risks associated with amniocentesis and CVS, they are not done routinely. The tests usually are offered to pregnant women with an increased risk for having a fetus with abnormal chromosomes. This includes women who have had an abnormal nuchal fold ultrasound or triple or quad screen test, who are age 35 or older, who have a child with Down syndrome, or who have a family history of the condition. The cost of a triple or quad screen test is not covered by all provincial health plans.
It is always your choice whether to have your fetus tested for Down syndrome. A diagnosis of Down syndrome made early in fetal development allows you the option of terminating your pregnancy. If termination is not an option for you, early awareness can help you prepare emotionally and plan for the special needs of your child. A genetic counsellor can help you during this process.
If a fetus is diagnosed with Down syndrome, a doctor may recommend fetal echocardiography to screen for heart defects and fetal ultrasound to help detect digestive system problems. Any suspected defects will be further investigated after birth.
Diagnosis after your baby is born
If Down syndrome was not diagnosed before birth with a karyotype test (using amniocentesis or chorionic villus sampling), a baby's physical features at birth often give doctors a clear sense of whether the child has Down syndrome. But traits can be subtle in a newborn, depending on the type of Down syndrome that he or she has. In general, a preliminary diagnosis can often be made from observation of physical features and:
- Results of earlier triple or quad screening tests (if any are available).
- Details about the parents' medical histories.
- Physical examination of the baby for other medical problems.
A newborn will also have a blood sample taken for chromosomal analysis to confirm the diagnosis. It may take 2 to 3 weeks to get the complete results of this test.
Testing at birth and throughout life
Screening guidelines were developed by the American Academy of Pediatrics and the U.K. Down Syndrome Medical Interest Group (DSMIG) for people with Down syndrome according to age. Doctors screen for common problems seen in people with Down syndrome, such as heart, hearing, or vision problems.
- Testing for newborns (birth to 1 month of age)
- Testing for infants (age 1 month to 1 year)
- Testing during early childhood (age 1 to 5 years)
- Testing during middle and late childhood (age 5 to 13 years)
- Testing during adolescence and young adulthood (age 13 to 21 years)
- Testing during adulthood
Early Detection
Testing for Down syndrome during pregnancy (prenatal testing), which requires your consent, may be recommended if you:
- Are 35 or older.
- Have a family history of Down syndrome or your partner has a family history of Down syndrome.
- Know that you or your partner carry the gene for the translocation type of Down syndrome.
Screening for Down syndrome is done with the maternal triple screen test. Prenatal testing for a formal diagnosis of Down syndrome requires karyotyping. The sample used for this testing is taken during amniocentesis or chorionic villus sampling.
Genetic counselling offers a lot of information and help for couples who are planning to become pregnant. For example, couples who have a child with Down syndrome can use genetic counselling to help determine their risk for having another child with the condition.
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| Author: | Debby Golonka, MPH Ralph Poore | Last Updated: October 4, 2007 |
| Medical Review: | Adam Husney, MD - Family Medicine Michael J. Sexton, MD - Pediatrics Donald Sproule, MD, CM, CCFP, FCFP - Family Medicine | |
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