How the Down syndrome test works and tips for making your decision

Down syndrome is a topic that comes up early in every pregnancy. Many of our patients are still in the afterglow of learning they are pregnant when we offer them a screening for this well-known genetic disorder.

Although we are fortunate that the technology exists to find out if a baby will have Down syndrome, some women struggle with the decision of whether to have the screening.

I’ll cut to the chase – there is no right or wrong answer. But there are things you should know and factors to consider.

This post explains how we test for Down syndrome and other chromosome abnormalities. Then, I will offer some advice about things to consider as you make your decision about the screening test.

How we test for Down syndrome

Patients in their first trimester are offered a screening to look for chromosome abnormalities. The screening cannot confirm that a baby has Down syndrome but rather looks for clues related to the condition. If the patient agrees to the screening, an ultrasound and a blood test are completed.

The screening must be performed when the patient is between 11 and 14 weeks pregnant. At this point in gestation, the base of the baby’s neck is still translucent, which allows the sonographer to take a specific measurement in this area.

Babies with Down syndrome and other conditions related to chromosome abnormalities usually have extra fluid in an area behind the neck during this stage of the pregnancy. In addition to Down syndrome, a larger-than-normal measurement can guide doctors toward other conditions that have no connection to chromosomes, such as heart abnormalities.

The neck measurement, the patient’s age, the baby’s gestational age, and the blood test all help the doctor determine the likelihood of a chromosome abnormality.

Other screening methods are available for patients who visit us during their second trimester.

If the screening indicates a strong likelihood of Down syndrome, we offer further diagnostic testing to confirm the diagnosis. Tests are available that analyze the amniotic fluid surrounding the baby, cells from the placenta, or blood from the umbilical cord. Just like the screening, these tests are optional. Because there is a risk of miscarriage with each of these tests, they are not offered unless the results of the screening are abnormal.

Testing options for ‘high-risk’ patients

Patients are more likely to have a baby with Down syndrome or another chromosome abnormality when they are age 35 or older, or if they have already had a child with such an abnormality. These patients are considered “high-risk” and have additional testing options.

One of the newest tests offered to high-risk patients is a blood test that compares DNA from the mom and the baby. It can be done as early as Week 10 of the pregnancy.

With each pregnancy, some DNA from the baby will find its way into the mother’s bloodstream. In a normal pregnancy, the ratio of chromosomes between the mother and baby is proportionate.

If the test results reveal a lot more fetal DNA on one of the chromosome pairs, we know there is a very strong likelihood the baby will have Down syndrome or other disorders linked to that chromosome abnormality.

How to decide whether to have the screening for Down syndrome

Deciding whether to have the screening for Down syndrome is a personal choice. My best advice is to think about what you would do with the results. If you wouldn’t take any action, regardless of the results, then the screening may be unnecessary.

However, if pregnancy termination is something you would consider, it’s probably a good idea to have the screening performed.

Some patients may want to know as soon as possible that their baby will have Down syndrome, so they have extra time to learn about the condition and seek available resources. Other patients may not have a desire to find out, for any number of reasons.

The mother’s age is another factor to consider. We know that the risk of having a baby born with Down syndrome increases as the mother gets older. In fact, a mother who is 35 years old is three times more likely to have a baby born with Down syndrome than a mother who is 25 years old.

A discussion with your doctor can help you make the best decision for your personal situation.

To learn more about the basics of Down syndrome and how it was first discovered, read this post from my colleague, Dr. Horsager.