Cell-free fetal DNA is a screening test to look for certain birth defects in a fetus. It's done to find birth defects caused by an abnormal number of chromosomes. It also can reveal the sex and blood type of the fetus.
This is a blood test for the mother. The test can be done as early as 10 weeks in the pregnancy.
If this screening test is positive, it means there is a chance your baby has a birth defect. In that case, your doctor may suggest that you have a diagnostic test, which can show if there is a birth defect.
Why It Is Done
The test is used to look for birth defects caused by too many or too few chromosomes, such as trisomy 18 and Down syndrome (trisomy 21). These birth defects are called trisomies (say "try-SOH-mees") because the fetus's cells have three copies of a chromosome instead of the usual two. For example, in trisomy 18—which causes physical problems and intellectual disability—there are three copies of chromosome 18 instead of two.
The test is available for women who are at high risk for having a baby with a birth defect. These include women who have any of the following risks:
- They are 35 or older.
- They have a child who has a birth defect.
- They had an ultrasound test that showed a possible birth defect.
The test is not recommended for women who are at low risk of having a baby with a chromosomal birth defect. Talk with your doctor about how high your risk is.
How To Prepare
There is nothing you need to do to prepare for this test.
How It Is Done
The health professional taking a sample of blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
How It Feels
An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
The fetal DNA will be looked at to see if there are missing or extra chromosomes. If there is an extra or missing chromosome (a positive result), your baby may have a chromosomal birth defect. A negative result means that your baby is unlikely to have one of the birth defects this test looks for.
This test can find more than 97% of trisomies such as 18 and 21. That means that it will find these birth defects more than 97 times out of 100 and won't find them less than 3 times out of 100.footnote 1
What Affects the Test
This test is not recommended for women who are pregnant with twins or more. It hasn't been well studied in this group of pregnant women.footnote 1
What To Think About
If you have a positive result (meaning there is a high chance that your baby has a birth defect), you can talk with your doctor and partner about whether you want to have a diagnostic test, such as chorionic villus sampling or amniocentesis. Either of these tests can show if your baby has certain birth defects.
Current as of: May 28, 2019
Author: Healthwise Staff
Kathleen Romito MD - Family Medicine
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Siobhan M. Dolan MD, MPH - Reproductive Genetics
Current as of: May 28, 2019