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).

How To Prepare

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How It Is Done

A health professional uses a needle to take a blood sample, usually from the arm.

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

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 up to 99% of trisomies such as 18 and 21. That means that it will find these birth defects up to 99 times out of 100 and won't find them 1 time out of 100.^{footnote 1}