Fetal DNA Tests
How and when is the cell-free DNA test performed?
Cell-free DNA is DNA released from the placenta and is found in a pregnant woman’s blood from 10 weeks onward in pregnancy. Some of the cells in the placenta (afterbirth) normally break down and release DNA into the mother’s bloodstream. The new screening test for Down syndrome measures the amount of DNA from chromosome 21 in a sample of a pregnant woman’s blood. Because cells of fetuses with Down syndrome have an extra 21st chromosome, they release more chromosome 21 DNA. This usually causes a higher than expected amount of chromosome 21 DNA in the mother’s blood.
How accurate is the cell-free DNA screening?
Approximately 99 percent of pregnancies with Down syndrome and trisomy 18 will have an abnormal cell-free DNA result. However, a small number of women have a false positive or false negative result. Positive results should be followed up with CVS or amniocentesis, especially if a patient would consider termination of a pregnancy with a chromosome abnormality.
What are the advantages associated with cell-free DNA screening?
Can women under age 35 choose to have cell-free DNA screening?
Yes, they can. However, many insurance companies will only cover the cost in high risk patients. Cell-free DNA screening appears to have a higher detection rate and lower false positive rate than the traditional screening tests for low risk patients.
If my cell-free DNA screening test is negative, do I need additional testing?
What is being tested?
Cell-free fetal DNA (cffDNA) is genetic material that is released by the placenta and circulates in a woman's blood during pregnancy. CffDNA generally reflects the genetic makeup of the developing baby (fetus). Cell-free fetal DNA prenatal screening, called non-invasive prenatal screening (NIPS) by some, detects defects in fetal DNA after it is purified from the pregnant woman's blood.
CffDNA is present in a pregnant woman's blood in small quantities starting in the first trimester and increases throughout pregnancy. The cffDNA test is a relatively new, non-invasive method of screening that can be performed as early as the tenth week of pregnancy for women.
The test can identify chromosome disorders in a developing baby, including the presence of extra chromosomes (trisomies) such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). The extra genetic material present in these conditions affects the development of the baby and causes characteristic signs, symptoms, and complications.
Down syndrome is the most common of the three trisomies detected by cffDNA testing and varies significantly in severity from person to person. Edwards syndrome and Patau syndrome are more rare and more severe, with most affected babies dying within weeks to months of birth. This test may be used to identify other rare conditions resulting from an extra chromosome or missing piece of chromosome (microdeletion).
CffDNA testing can also detect an extra sex chromosome. One example is Klinefelter syndrome, resulting from two X chromosomes and one Y chromosome. For more on this and other rare chromosome disorders, see the Related Pages section of this article.
A blood sample is obtained by inserting a needle into a vein in the mother's arm.
No test preparation is needed.