Jordan Ellis
Down syndrome is a genetic condition caused by an extra copy of chromosome 21, resulting in certain characteristics, including cognitive disabilities, developmental delays, and distinct physical features. It is the most common chromosomal condition diagnosed in the United States, with about 5,700 babies born with Down syndrome each year. Hospitals use various methods to test newborns for Down syndrome, including percutaneous umbilical blood sampling (PUBS), amniocentesis, and chorionic villus sampling (CVS). These tests are typically performed during pregnancy to estimate the risk or diagnose Down syndrome, but they can also be used after birth to confirm the condition in newborns exhibiting signs of Down syndrome. While these tests provide valuable information, they carry different risks, and it is important for parents to discuss these risks and benefits with healthcare providers before deciding on testing options.
| Characteristics | Values |
|---|---|
| Down syndrome definition | Down syndrome is a genetic condition caused by an extra copy of chromosome 21, resulting in certain characteristics, including cognitive disability, developmental delays, and distinct physical traits. |
| Incidence | About 1 in 1,000 births in the United States; the chance of having a baby with Down syndrome increases with the pregnant person's age. |
| Types of Down syndrome | Trisomy 21 (95% of cases), Translocation (3-4% of cases), and Mosaicism. |
| Testing methods | Screening tests (blood test, ultrasound/nuchal translucency measurement, prenatal screening) and diagnostic tests (amniocentesis, chorionic villus sampling, percutaneous umbilical blood sampling). |
| Testing timing | <co: 9,10,16,18>Screening tests are usually done during the first trimester, while diagnostic tests can be done later in pregnancy, typically between weeks 9 and 22. |
| Test results | Screening tests provide an increased likelihood of Down syndrome, while diagnostic tests can confirm the presence of Down syndrome with more certainty but carry a slightly higher risk of miscarriage. |
| Test costs | The combined first-trimester blood test is covered by Medicare, while the NIPT blood test costs around $400 to $500. |
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Screening tests
There are several screening tests available to detect Down syndrome during pregnancy. The most common approach is the combined first-trimester screening, which includes a blood test and an ultrasound. The blood test checks for “markers,” such as certain proteins, in the mother's blood that suggest an increased likelihood of Down syndrome. The ultrasound, called the measurement of nuchal translucency, detects fluid at the back of the fetus's neck, which can sometimes indicate Down syndrome. This combined method has higher detection rates than tests performed during the second trimester.
Another screening test is the cell-free fetal DNA test, which checks the mother's blood for unusually large amounts of material from chromosome 21. This test can be done as early as ten weeks into the pregnancy but is not recommended for those with a low risk of having a baby with Down syndrome or those pregnant with multiple babies.
If a screening test indicates a high likelihood of Down syndrome, a diagnostic test, such as amniocentesis or chorionic villus sampling (CVS), can be performed to confirm the diagnosis. These diagnostic tests carry a slightly higher risk to the fetus than screening tests and are often done after a positive screening test result.
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Diagnostic tests
Down syndrome is a genetic condition caused by extra copies of chromosome 21. It results in certain characteristics, including some degree of cognitive disability and other developmental delays. The incidence of Down syndrome in the United States is about 1 in 1,000 births.
There are two types of tests available to detect Down syndrome during pregnancy: screening tests and diagnostic tests. Screening tests can indicate a higher likelihood of Down syndrome but cannot provide a definitive diagnosis. On the other hand, diagnostic tests can confirm the presence of Down syndrome but carry a slightly higher risk of miscarriage.
Amniocentesis
Amniocentesis is a procedure where a small sample of amniotic fluid is extracted from the uterus. This test is typically performed between weeks 14 and 18 of pregnancy. It carries a slight risk of miscarriage and can cause mild discomfort.
Chorionic Villus Sampling (CVS)
CVS involves taking a sample of cells from the placenta, which connects the mother and fetus. This procedure is done between weeks 9 and 13 of pregnancy and can detect chromosomal abnormalities earlier than amniocentesis. CVS is performed under ultrasound guidance, using either a needle inserted through the abdomen or a catheter through the vagina and cervix.
Percutaneous Umbilical Blood Sampling (PUBS)
PUBS, also known as cordocentesis, involves taking a sample of fetal blood from the umbilical cord. This test provides the most accurate diagnosis of Down syndrome during pregnancy but is not commonly used due to higher risks compared to other tests. It is usually performed between weeks 18 and 22.
These diagnostic tests can provide a certain diagnosis of Down syndrome, but they are not without risks. It is important for parents to discuss these tests with their healthcare providers, weighing the benefits and risks before making an informed decision.
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Amniocentesis
The procedure may cause mild discomfort, such as a stinging or cramping sensation, and there is a slight risk of miscarriage, estimated at less than 1% in the second trimester. However, amniocentesis is generally considered safe, and the American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women have the choice to undergo prenatal tests like amniocentesis.
Amniotic fluid contains cells shed by the fetus, which carry genetic information. By analysing these cells, healthcare providers can detect chromosomal abnormalities associated with Down syndrome, such as an extra copy of chromosome 21, also known as trisomy 21. Amniocentesis can also be used to diagnose other genetic disorders, such as cystic fibrosis or Tay-Sachs disease, as well as neural tube defects like spina bifida.
It is important to note that amniocentesis is not the only method for diagnosing Down syndrome. Screening tests, which indicate the risk of Down syndrome, are typically performed before diagnostic tests. Additionally, other diagnostic tests, such as percutaneous umbilical blood sampling (PUBS), can also be used to confirm a diagnosis of Down syndrome. However, PUBS is generally performed later in pregnancy and carries a slightly higher risk of causing serious problems compared to amniocentesis.
Before undergoing amniocentesis, it is essential to discuss any concerns, risks, and preparation instructions with a healthcare provider. This includes informing the provider about any medications, supplements, or allergies, as well as disclosing one's blood type, as Rh sensitization can occur if the mother is Rh negative and the fetus is Rh positive.
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Chorionic villus sampling
The procedure can be done via two methods: transabdominal CVS or transcervical CVS. In transabdominal CVS, a needle is inserted through the abdomen to reach the placenta, while in transcervical CVS, a tube or small forceps are inserted through the cervix to access the placental tissue. The test itself takes about 10 minutes, but the entire consultation may take up to 30 minutes. Patients may experience mild discomfort and cramping during and after the procedure, similar to period pains.
The benefits of CVS include its ability to provide diagnostic results earlier than other tests such as amniocentesis, which is typically performed after 15 weeks of gestation. This early diagnosis allows for more clinical decision-making options during the first trimester. However, CVS does not provide information on neural tube defects like spina bifida, so a follow-up blood test between 16 and 18 weeks of pregnancy is often necessary.
While CVS is considered generally safe, it does carry a small risk of complications, including miscarriage. Therefore, informed patient counseling is essential before the procedure. CVS is usually recommended for individuals with an increased risk of genetic or chromosomal conditions, including abnormal screening results, advanced maternal age, or a family history of genetic disorders.
After the CVS procedure, the patient's vital signs and the baby's heart rate will be monitored for at least an hour. The placental tissue sample is sent to a genetics lab for analysis, and results are typically available within 3 days for the rapid CVS result and after 2 weeks for a more detailed report. Depending on the results, genetic counseling with a specialist may be advised.
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Ultrasound
The combined first-trimester screening test is the most accepted approach for detecting Down syndrome during the first trimester. The results from the blood test and ultrasound are combined to produce one Down syndrome risk rating. The first-trimester combined screening test results in more effective or comparable detection rates than methods used during the second trimester.
If the screening test shows an increased likelihood of Down syndrome, a diagnostic test can be ordered to confirm the diagnosis. Diagnostic tests carry a slightly greater risk to the fetus than screening tests. Diagnostic tests involve removing a sample of genetic material, which is then checked for extra material from chromosome 21, which may indicate that a fetus has Down syndrome.
Chorionic villus sampling (CVS) is a diagnostic procedure that can be performed under ultrasound guidance. CVS involves removing a small piece of tissue from the placenta, which can be obtained with a needle inserted through the abdomen or a catheter inserted through the vagina and into the cervix. The tissue is then cultured and tested for chromosomal abnormalities. CVS can be performed much earlier in pregnancy than other diagnostic tests, usually between 10 and 13 weeks, providing information earlier in pregnancy.
While ultrasound tests are an important tool for screening and diagnosing Down syndrome, they are just one of several options available to healthcare providers. Other screening and diagnostic tests include blood tests, amniocentesis, percutaneous umbilical blood sampling (PUBS), and cell-free fetal DNA tests.
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Frequently asked questions
Down syndrome is a genetic condition where a person is born with an extra chromosome, typically chromosome 21, which affects how their brain and body develop. Distinct physical signs of Down syndrome are usually present at birth and become more apparent as the baby grows.
After birth, a karyotype test is typically performed on a sample of the baby's blood to check the number and structure of the baby's chromosomes.
During pregnancy, there are two types of tests for Down syndrome: screening tests and diagnostic tests. Screening tests can indicate an increased likelihood of Down syndrome, while diagnostic tests can confirm its presence. Screening tests include blood tests and ultrasounds, while diagnostic tests include procedures such as amniocentesis and chorionic villus sampling.
The chance of having a baby with Down syndrome is related to the mother's age. For women under 25, the odds are about 1 in 1,400. At age 35, the odds are about 1 in 350, and at age 40, they increase to about 1 in 100. However, the majority of babies with Down syndrome are born to mothers under 35 due to the higher number of births among younger women.
