Screening for Down Syndrome During Pregnancy

The Concepts Behind Down Syndrome Screening Tests

doctor performing ultrasound on pregnant woman
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The Concept of Screening for Down Syndrome During Pregnancy

The number of screening options for Down syndrome has increased dramatically in the last few years. Before you can make a decision about what testing, if any, is right for you, it is important to understand the concept behind screening tests.

Screening and screening tests can be difficult concepts for people to understand. We are used to medical tests giving us an answer, but with screening tests, instead of an answer, we get an estimate of risk.

For example, a screening tests cannot tell you for sure that your baby has Down syndrome, it can only give you an estimate of your risk to have a baby with Down syndrome. Based upon this risk estimate and a predetermined risk cutoff, your pregnancy will be classified as screen negative (low-risk) or screen positive (high-risk). Basically screening separates people into two populations-–those that are deemed low-risk (the majority) and those that are deemed high-risk (a minority).

This might sound a little complicated but I think that looking at a simplified example will help.

An Example of A Screening Test

One simple screening test that can assess a mother’s risk to have a child with Down syndrome is simply to ask a mother-to-be her age. Based on her answer and a risk-cutoff, mothers-to-be can be separated into two groups--those who are considered low-risk (screen negative) and those who are considered high-risk (screen positive).

To separate screen positive mothers from screen negative mothers, let’s pretend that anybody with a risk of greater than 1 in 200 (or one-half of 1 percent) is considered screen positive. This 1 in 200 risk is our risk cut-off.

Now, let’s ask two mothers-to-be their ages. Mom A is 30 years old and based on her age alone, her risk to have a baby with Down syndrome is 1 in 900.

She is considered “screen negative” since her risk is less than our cut-off risk of 1 in 200. So her risk is low and she wouldn’t be offered any follow-up testing. But, and this is a big but, her risk is not zero--it is 1 in 900. That means that if 900 30-year-old moms-to-be were in a room, one would have a baby with Down syndrome even though our “test” said she was screen negative (low risk!

Now let’s ask Mom B her age. Mom B is 38 and based on her age alone, her risk to have a baby with Down syndrome is 1 in 180 (or a little greater than our risk cutoff of 1 in 200). Since her risk is greater than 1 in 200, she is considered “screen positive” or high-risk. Now obviously, her risk is still about one half of one percent (or greater than 99% chance that her fetus does not have Down syndrome) but according to our test, her result is “screen positive.” While she is considered “screen positive,” it is still more likely that her baby does not have Down syndrome. However, based on her “risk,” she would be offered follow-up diagnostic testing to determine if the baby has Down syndrome.

Most women, even with a positive screening result, will have babies that do not have Down syndrome. You can see however, that getting a “screen positive” result could raise your anxiety.

Advantages and Disadvantages of Screening Tests

While screening tests don’t tell you for sure about your baby’s chromosomes, they do have some advantages compared to diagnostic testing such as amniocentesis or CVS. For one, there is no risk to the pregnancy. Most screening tests are either blood tests or ultrasounds or a combination of both, and thus there is no risk of miscarriage associated with them. The disadvantage is that they don’t give you a firm answer, they just give you an estimate of your risk. Most often this estimate is low (screen negative) and many women find this reassuring. However, if your screening result is considered positive, this may cause you a great deal of anxiety even though it is most likely that your baby does not have Down syndrome. If your tests are considered screen positive, you will also be faced with making a choice about diagnostic testing.

The Bottom Line

The decision to have prenatal testing during pregnancy is a personal one. Most screening tests provide parents-to-be with reassurance. However, when a screening test is screen positive, it can be anxiety-provoking. Follow-up diagnostic testing is available, but it has some risks associated with it and it takes some time to get the results, which can be hard for some parents-to-be. In making a decision about any form of prenatal testing during pregnancy, it is important to consider what the results of the test mean for you and what you would do with that information.


American College of Obstetricians and Gynecologists (ACOG). Screening for Fetal Chromosomal Abnormalities. ACOG Practice Bulletin, number 77, January 2007.

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