How is down syndrome (trisomy) diagnose? Prenatal Screening for down syndrome is crucial during a woman's pregnancy
All pregnant women, no matter what their age, should be given the choice to have screening for genetic conditions such as Down’s syndrome.
Antenatal screening is a way of assessing whether your unborn baby could develop, or has developed, an abnormality during your pregnancy.
Antenatal screening cannot diagnose conditions such as Down’s syndrome, but it can determine the likelihood of your baby developing the syndrome.
If the risk of Down’s syndrome (or any other condition) is shown to be high, further testing can be arranged to confirm whether or not your baby has the condition. The test that is used for Down’s syndrome is a combined blood test and an ultrasound scan, which is known as the ‘combined test’.
More: What is Trisomy
Various screening tests can help identify whether you have a high risk of carrying a baby with Down syndrome. In the past, blood tests typically have been offered around the 16th week of pregnancy to screen for Down syndrome, spina bifida and various other chromosomal disorders.
Currently, more women are electing the first trimester combined test, done in two steps during week 11 to 13 of pregnancy. The first trimester combined test includes:
Ultrasound. The doctor uses ultrasound to measure a specific region on the back of a baby's neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this tissue.
Blood tests. Results of the ultrasound are paired with blood tests that measure levels of pregnancy-associated plasma protein-A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
If early assessment isn't your first priority, you can have full integrated testing, which is done in two parts during the first two trimesters of your pregnancy. The results of the two parts are combined to estimate the risk that your baby has Down syndrome. This test can achieve the same level of detection as the first trimester combined test but with a lower false-positive rate, meaning that fewer women are incorrectly identified as carrying a baby with Down syndrome.
First trimester. Part one includes an ultrasound to measure nuchal translucency and a blood test to measure PAPP-A.
Second trimester. Done at 15 to 20 weeks of pregnancy, the quad screen measures your blood level of four pregnancy-associated substances, alpha fetoprotein, estriol, HCG and inhibin A.
Of all the women who undergo screening tests for Down syndrome, about 5 percent are identified as being at risk. But the overall risk of Down syndrome among pregnant women is far below 5 percent. So the screening tests, by design, are much better at identifying women who may be at risk of having a baby with Down syndrome than women who actually have a baby with Down syndrome.
If a screening test indicates a high risk of Down syndrome, a more invasive test may be used to determine whether your baby actually has Down syndrome (diagnostic test).
Diagnostic tests during pregnancy
If your screening test results are positive or worrisome or you're at high risk of having a baby with Down syndrome, you might consider further testing to confirm the diagnosis. Diagnostic tests that can identify Down syndrome include:
Amniocentesis. A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus. This sample is then used to analyze the chromosomes of the fetus. Doctors usually perform this test after 15 weeks of gestation.
Chorionic villus sampling (CVS). Cells taken from the mother's placenta can be used to analyze the fetal chromosomes. Typically performed between the ninth and 14th week of pregnancy, this test carries a 1 in 100 risk of miscarriage.
Percutaneous umbilical blood sampling (PUBS). Blood is taken from a vein in the umbilical cord and examined for chromosomal defects. Doctors generally perform this test after 18 weeks of gestation. This test carries a greater risk of miscarriage than does amniocentesis or chorionic villus sampling. Generally, this test is only done when results of other tests are unclear.
Researchers are working on improved ways of detecting genetic problems early on, including:
Preimplantation genetic diagnosis. One option that may become available for couples undergoing in vitro fertilization is testing of the embryo for genetic abnormalities before it's implanted in the womb.
Analysis of circulating fetal DNA. Although not widely available, new tests that evaluate fetal DNA circulating in the mother's blood may create another option for prenatal diagnosis of Down syndrome and other chromosomal abnormalities.
After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. If your child displays some or all of the characteristics of Down syndrome, your doctor probably will order a test called a chromosomal karyotype. This test is an analysis of your child's chromosomes. If there's an extra chromosome 21 present in all or some of the cells, the diagnosis is Down syndrome.
Screening tests during pregnancy
Various screening tests can help identify whether you have a high risk of carrying a baby with Down syndrome. In the past, blood tests typically have been offered around the 16th week of pregnancy to screen for Down syndrome, spina bifida and various other chromosomal disorders.
Currently, more women are electing the first trimester combined test, done in two steps during week 11 to 13 of pregnancy. The first trimester combined test includes:
Ultrasound. The doctor uses ultrasound to measure a specific region on the back of a baby's neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this tissue.
The primary purpose of ultrasound is to determine the status of a pregnancy -- the due date, size of the fetus and multiple gestation. Ultrasound also can provide some information about possible birth defects in a fetus. Results of the ultrasound are explained at the time of the visit. In some patients, an ultrasound raises concern of a possible abnormality in the fetus.
Blood tests. Results of the ultrasound are paired with blood tests that measure levels of pregnancy-associated plasma protein-A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
If early assessment isn't your first priority, you can have full integrated testing, which is done in two parts during the first two trimesters of your pregnancy. The results of the two parts are combined to estimate the risk that your baby has Down syndrome. This test can achieve the same level of detection as the first trimester combined test but with a lower false-positive rate, meaning that fewer women are incorrectly identified as carrying a baby with Down syndrome.
First trimester. Part one includes an ultrasound to measure nuchal translucency and a blood test to measure PAPP-A.
Second trimester. Done at 15 to 20 weeks of pregnancy, the quad screen measures your blood level of four pregnancy-associated substances, alpha fetoprotein, estriol, HCG and inhibin A.
Of all the women who undergo screening tests for Down syndrome, about 5 percent are identified as being at risk. But the overall risk of Down syndrome among pregnant women is far below 5 percent. So the screening tests, by design, are much better at identifying women who may be at risk of having a baby with Down syndrome than women who actually have a baby with Down syndrome.
If a screening test indicates a high risk of Down syndrome, a more invasive test may be used to determine whether your baby actually has Down syndrome (diagnostic test).
Diagnostic tests during pregnancy
If your screening test results are positive or worrisome or you're at high risk of having a baby with Down syndrome, you might consider further testing to confirm the diagnosis. Diagnostic tests that can identify Down syndrome include:
Amniocentesis. A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus. This sample is then used to analyze the chromosomes of the fetus. Doctors usually perform this test after 15 weeks of gestation.
Amniocentesis is used most commonly to identify chromosomal problems, such as Down syndrome. When the fetus is known to be at risk, it can detect other genetic diseases like cystic fibrosis, Tay-Sachs disease and sickle cell disease. An amniocentesis procedure for genetic testing is typically performed between 15 and 20 weeks of pregnancy. Under ultrasound guidance, a needle is inserted through the abdomen to remove a small amount of amniotic fluid. The cells from the fluid are then cultured and a karyotype analysis — an analysis of the chromosomal make-up of the cells — is performed. It takes about two weeks to receive the results of the test.Amniocentesis detects most chromosomal disorders, such as Down syndrome, with a high degree of accuracy. Testing for other genetic diseases, such as Tay-Sachs disease, is not routinely performed but can be detected through specialized testing if your fetus is known to be at risk.Testing for neural tube defects, such as spina bifida, also can be performed. There is a small risk of miscarriage as a result of amniocentesis — about 1 in 100 or less. Miscarriage rates for procedures performed at UCSF Medical Center are less than 1 in 350.
Chorionic villus sampling (CVS). Cells taken from the mother's placenta can be used to analyze the fetal chromosomes. Typically performed between the ninth and 14th week of pregnancy, this test carries a 1 in 100 risk of miscarriage.
Chronic Villus Sampling (CVS) Like amniocentesis, chronic villus sampling is used most commonly to identify chromosomal problems, such as Down syndrome. It can detect other genetic diseases like cystic fibrosis, Tay-Sachs disease and sickle cell disease in at-risk fetuses. The main advantage of CVS over amniocentesis is that it is done much earlier in pregnancy, at 10 to 12 weeks, rather than 15 to 20 weeks.CVS involve removing a tiny piece of tissue from the placenta. Under ultrasound guidance, the tissue is obtained either with a needle through the abdomen or a catheter inserted through the cervix. The tissue is then cultured and a karyotype analysis &mash; analysis of the chromosomal make-up of the cells — is performed. It takes about two weeks to receive the results. The advantage of CVS over amniocentesis is that the test is performed much earlier in pregnancy, so results are typically available by the end of the third month. A disadvantage is that spinal cord defects cannot be detected. Expanded alpha fetoprotein (AFP) blood testing or ultrasound can be performed later in the pregnancy to screen for spinal cord defects. There is a small risk of miscarriage as a result of CVS -- 1 in 100 or less. Miscarriage rates for procedures performed at UCSF Medical Center are less than 1 in 350.
Percutaneous umbilical blood sampling (PUBS). Blood is taken from a vein in the umbilical cord and examined for chromosomal defects. Doctors generally perform this test after 18 weeks of gestation. This test carries a greater risk of miscarriage than does amniocentesis or chorionic villus sampling. Generally, this test is only done when results of other tests are unclear.
New prenatal tests under study
Researchers are working on improved ways of detecting genetic problems early on, including:
Preimplantation genetic diagnosis. One option that may become available for couples undergoing in vitro fertilization is testing of the embryo for genetic abnormalities before it's implanted in the womb.
Analysis of circulating fetal DNA. Although not widely available, new tests that evaluate fetal DNA circulating in the mother's blood may create another option for prenatal diagnosis of Down syndrome and other chromosomal abnormalities.
Diagnostic tests for newborns
After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. If your child displays some or all of the characteristics of Down syndrome, your doctor probably will order a test called a chromosomal karyotype. This test is an analysis of your child's chromosomes. If there's an extra chromosome 21 present in all or some of the cells, the diagnosis is Down syndrome.
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