What is a cell-free DNA test in pregnancy?

Did you know that fetal cells enter the mother's blood as early as 8-10 weeks of the pregnancy? This is where the cell-free fetal DNA test comes in, a not-so-new blood screening test where a sample of the woman's blood is taken after 10 weeks of the pregnancy to screen for certain conditions in the fetus such as Down syndrome or other chromosomal issues. In addition, it can tell you the fetus' sex.

The cell-free DNA test is a "non-invasive" test which requires only a blood test. The cell-free DNA test is a screening test, not a diagnostic test. If provides risks for certain conditions but does not make a definite diagnosis. If it's abnormal, you need additional tests such as chorionic villi sampling (CVS) or amniocentesis to confirm the diagnosis.

The test measures the relative amount of free fetal DNA in the mother's blood and indicates if a woman is at increased risk of having a fetus with Down syndrome (trisomy 21), Edward syndrome (trisomy 18) and Patau syndrome (trisomy 13).  The test determines the chance that the fetus has Down syndrome, Edward syndrome or Patau syndrome based on the relative amount of DNA from chromosomes 21, 18 and 13. usually additional tests are required like a chorionic villi sampling or CVS or an amniocentesis to determine the actual diagnosis. 

The results are usually available within one to two weeks of the blood draw.

Cell-free fetal DNA testing is thought to detect greater than 99 percent of all Down syndrome pregnancies and greater than 98 percent of all trisomy 18 pregnancies. It detects about 65 percent of all trisomy 13 pregnancies.

Cell-free fetal DNA testing is not designed for use in women with multiple pregnancies such as twins, or in women with egg donor pregnancies.

Results of the cell-free DNA test 

A cell-free DNA test is a screening test. You usually receive a percent (%) risk for the condition, not the absolute diagnosis. To make a diagnosis, you need additional tests such as a CVS or amniocentesis.