Cell-Free Fetal DNA Test

Test Overview

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:

  • Clean the needle site with alcohol.

How It Feels






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.



  1. American College of Obstetricians and Gynecologists (2015). Cell-free DNA screening for fetal aneuploidy. ACOG Committee Opinion No. 640. Obstetrics and Gynecology, 126(3): 691–692. DOI: 10.1097/01.AOG.0000471171.86798.ac. Accessed April 11, 2017.


Current as ofSeptember 5, 2018

Author: Healthwise Staff
Medical Review: Kathleen Romito, MD - Family Medicine
E. Gregory Thompson, MD - Internal Medicine
Adam Husney, MD - Family Medicine
Siobhan M. Dolan, MD, MPH - Reproductive Genetics

Cell-Free Fetal DNA Test