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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?

Cell-free DNA screening has a higher detection rate and a lower false positive rate than the sequential screen and MMS.

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?

Yes. Screening for open neural tube defects and an ultrasound at 20 weeks gestation are recommended for all women.

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