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Tests such as NIPT, first-trimester screening, and second-trimester screening can give you an idea of your chances of having a baby with Down syndrome. If your test is positive, you can choose to have a diagnostic test such as CVS or amniocentesis to confirm the results.
Why should I consider screening for Down syndrome?
Many parents-to-be worry that their developing baby may have Down syndrome or some other chromosomal abnormality. Screening tests help you assess your baby's chances of having this kind of condition. The results can help you decide whether to have invasive diagnostic testing, such as CVS or amniocentesis, to find out for sure about your baby's condition.
What's a chromosomal abnormality?
Chromosomes are threadlike structures in each cell that carry genes. Most people have 46 chromosomes in each cell, with one set of 23 coming from the mother's egg and the other set from the father's sperm.
Biological errors can happen during the early stages of cell division, causing abnormalities in the chromosomes. For example, some babies develop with 47 chromosomes: Instead of 23 matched pairs, they have 22 pairs plus one set of three. This chromosomal abnormality is called trisomy.
Often, a woman who conceives a baby with an abnormal number of chromosomes will miscarry, usually early in pregnancy. But with certain chromosomal abnormalities, the baby may survive and be born with developmental problems and birth defects that can range from mild to severe.
Down syndrome, also known as trisomy 21, happens when a baby has an extra copy of chromosome 21 in some or all of his cells. Down syndrome is the most common chromosomal abnormality babies are born with.
Other chromosomal problems that babies can be born with include trisomy 18 and trisomy 13. These abnormalities are almost always associated with profound intellectual disability and other congenital malformations. If they survive to birth, these babies rarely live more than a few hours. Occasionally, they can live for a few months or a few years.
Are older moms more likely to have a baby with Down syndrome?
Yes. While anyone can have a baby with a chromosomal abnormality, the risk rises with the mother's age. For example, your likelihood of carrying a baby with Down syndrome increases from about 1 in 1,200 at age 25 to about 1 in 100 at age 40.
Are there risks to screening?
No, screens don't pose any risk to you or your baby because they aren't invasive: They don't involve inserting a needle into your uterus, for example. Instead, they use maternal blood samples and ultrasound measurements to assess your baby's chances of having certain chromosomal problems, including Down syndrome.
What screening tests are available for Down syndrome?
There are a variety of approaches to screening. Some tests are carried out in the first trimester and some in the second. They may include blood tests, an ultrasound scan, or, ideally, a combination of the two.
Different tests have different detection rates. The detection rate tells you how good the test is at identifying babies who are truly affected as being high risk.
NIPT (Noninvasive prenatal testing)
NIPT is a blood test that can detect Down syndrome and a few other chromosomal conditions at 10 weeks of pregnancy or later. NIPT is available to all women but has mainly been tested on women at high risk of having a baby with a chromosomal condition.
Although it's considered a screening test, NIPT has the best detection rate of all screening tests at 99 percent. If the results indicate a problem, it's still recommended to undergo CVS or amniocentesis for a definitive diagnosis. See more information on the NIPT (also known as cell-free fetal DNA testing).
First-trimester screening includes a blood test that measures the levels of two proteins in your blood and a special ultrasound scan called a nuchal translucency (NT) test. It can be done from 10 weeks through the end of 13 weeks.
The first-trimester blood test measures two proteins that are produced by your placenta: free beta-hCG and PAPP-A. A woman who is carrying a baby with Down syndrome is more likely to have abnormal levels of these two proteins in her blood.
The NT scan measures the clear (translucent) space in the tissue at the back of your baby's neck. Babies with chromosomal abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger.
Together, these tests are known as the first-trimester combined screening. They will identify as high risk about 82 to 87 percent of babies with Down syndrome (depending on which study you look at). This means that up to 18 percent of babies with Down syndrome will not be identified by this test.
The advantage of first-trimester screening is that you learn about your baby's risk for chromosomal problems relatively early in the pregnancy. If the risk is low, the results will offer you some reassurance.
If the risk is high, you may have the opportunity to consider CVS, which will allow you to find out whether your baby has a problem while you're still in your first trimester. CVS can be done any time after 10 weeks.
Second-trimester screening involves a blood test, commonly known as the multiple marker or the quadruple screen. This test is usually done between 15 and 22 weeks of pregnancy.
The test measures the levels of four substances in your blood: AFP, hCG, estriol, and inhibin A. Having abnormal levels of these substances in your blood is associated with an increased likelihood of carrying a baby with Down syndrome. The test detects about 81 percent of babies with Down syndrome.
Second-trimester screening is generally offered in conjunction with first-trimester screening as part of what's known as integrated or sequential screening (see below).
Integrated and sequential screening
Integrated and sequential screening involve elements of both first- and second-trimester screening tests, and they may improve the detection rate for Down syndrome.
How do I interpret the results of the screening test?
Screening tests provide you with a ratio that expresses your baby's chances of having Down syndrome, based on your test results and how old you are. This information may help you decide whether to undergo diagnostic testing.
The ratio you receive expresses your baby's chances of having Down syndrome. A risk of 1 in 100 means that for every 100 women with this result, one baby will have Down syndrome and 99 will not. A risk of 1 in 1,200 means that for every 1,200 women with this result, one baby will have Down syndrome and 1,199 will not. The higher the second number, the lower the risk.
You may also be told that your results are "normal" or "abnormal," depending on whether they are below or above the cutoff for that particular test.
For example, some tests use a cut off of 1 in 250. So a result of 1 in 1,200 would be considered normal because the risk that there's a problem is lower than 1 in 250. A result of 1 in 100 would be considered abnormal because that risk is higher than 1 in 250.
A normal test result (screen negative) does suggest that a problem is unlikely, but it doesn't guarantee that your baby has normal chromosomes. These screening tests are designed to look for only a few common chromosomal conditions and neural tube defects. A baby whose screen is negative may still have some other genetic problem or birth defect. In addition, a normal result doesn't ensure that a baby will have a "normal" IQ or brain function and doesn't rule out conditions like autism.
An abnormal test result (screen positive) doesn't mean that your baby has a chromosomal problem – just that she's more likely to have one. In fact, most screen-positive babies turn out not to have a problem.
If my screen is positive, how can I find out for sure if my baby has Down syndrome?
Diagnostic tests such as chorionic villus sampling (CVS) and amniocentesis can tell you for certain whether your child has a chromosomal abnormality. These tests can identify several hundred genetic disorders by analyzing the genetic makeup of cells from your baby or placenta. However, both procedures carry a small risk of miscarriage.
What else can screening tests tell me?
Screening tests can identify babies at higher risk for a few problems in addition to Down syndrome:
• First-trimester combined screening can detect trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome).
• An abnormal NT scan result can also indicate certain birth defects, such as major congenital heart problems and abdominal wall defects.
• Second-trimester screening can detect trisomy 18, and the AFP portion of the blood test screens for neural tube defects, such as spina bifida and anencephaly. (These are birth defects of the spinal cord and brain.)