Screening for Down Syndrome During Pregnancy

The Concepts Behind Down Syndrome Screening Tests

doctor performing ultrasound on pregnant woman
What does high risk and low risk mean in screening tests for Down syndrome in pregnancy?. gchutka E+/Getty Images

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 percent 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 prenatal 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 chorionic villi sampling (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 as there is with amniocentesis or CVS. 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.

Steps in Making a Decision About Prenatal Testing

As you think about your decisions, take a moment to go through these steps, considering what the next step would be no matter what testing you choose to have:

  • If you have an increased risk of Down syndrome based on your age, take a moment to understand how the risk of Down syndrome increases with age. As noted, even older mothers, say, over the age of 40, are more likely to have a baby without Down syndrome than a baby who has Down syndrome.
  • If, based on your age, you are concerned that your baby has an increased risk of Down syndrome, you then need to consider prenatal screening tests (either first trimester serum screening tests, B-HCG level, apha fetoprotein screening, ultrasound with first trimester nuchal fold testing or a level II ultrasound in mid-pregnancy?) Before having these tests done, consider the next step. If your non-invasive screening tests (blood tests or ultrasound) suggest you are in the high risk category rather than low risk, what would you do next? If you would wish to proceed with an amniocentesis or CVS, this is important to consider ahead of time. If you would not proceed with an amniocentesis or CVS, consider how you would cope with knowing you have an increased risk during your pregnancy. There is no right or wrong answer. Some people would be bothered to be aware of an increased risk whereas others would like to know if they had an increased risk in order to prepare for the possibility.
  • If you have an abnormal screening test (or due to an increased risk related to age) would you consider doing an amniocentesis or CVS? With these studies, it is important to weigh the knowledge you would gain against the risk of side effects such as miscarriage. While the risk of miscarriage with amniocentesis is low, it is still present. Yet before having this test consider what you would do knowing the results and if it would benefit you. Would you do anything different, for example, consider an abortion? Would knowing you were to have a baby with Down syndrome help you prepare better for the birth? Or, instead, are you someone who would fret through your pregnancy, finding that the diagnosis does not prepare you, but instead only makes you anxious?

The Bottom Line on Prenatal Testing for Down Syndrome

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.

Sources:

Halliday, J., Messerlian, G., and G. Palomaki. Patient education: Should I have a screening test for Down syndrome during pregnancy? (Beyond the Basics). UpToDate. Updated 08/10/15.

Continue Reading