Prenatal cell-free DNA screening

Overview

Prenatal cell-free DNA (cfDNA) screening, also known as noninvasive prenatal screening, is a method to screen for certain chromosomal abnormalities in a developing baby.

During prenatal cell-free DNA screening, DNA from the mother and fetus is extracted from a maternal blood sample and screened for the increased chance for specific chromosome problems, such as Down syndrome, trisomy 13 and trisomy 18. This screening can also provide information about fetal sex and rhesus (Rh) blood type.

Prenatal cell-free DNA screening is recommended for women who are at least 10 weeks pregnant and have adequate counseling regarding the options, benefits and limits of first and second trimester screening. Your health care provider or a genetic counselor will discuss whether prenatal cell-free DNA screening might benefit you and how to interpret the results.

Why it's done

Prenatal cell-free DNA screening is available to anyone who is pregnant. It can be used to screen for certain chromosomal disorders, including:

• Down syndrome (trisomy 21)
• Trisomy 18
• Trisomy 13

It can also be used to screen for fetal rhesus (Rh) blood type and fetal sex.

Some prenatal cell-free DNA screening tests might also screen for the increased chance for:

• Trisomy 16
• Trisomy 22
• Triploidy
• Sex chromosome aneuploidy
• Certain disorders caused by a chromosomal deletion (microdeletion syndrome), such as Prader-Willi syndrome
• Certain single-gene disorders associated with abnormalities of the skeleton or bones

Prenatal cell-free DNA screening might be more sensitive and specific than traditional first and second trimester screening, such as the first trimester screening and the quad screen. In addition, prenatal cell-free DNA screening might help women who have certain risk factors make decisions about invasive testing that carries a slight risk of miscarriage, including amniocentesis and chorionic villus sampling (CVS).

Keep in mind, however, that prenatal cell-free DNA screening has been shown to be less effective if you are:

• Pregnant with multiples
• Obese
• Pregnant via a donor egg
• Pregnant and a gestational carrier
• Less than 10 weeks pregnant
• Taking certain blood thinners

If you are obese, your health care provider will discuss other screening methods with you.

Risks

Prenatal cell-free DNA screening poses no physical risks for you or your baby.

While prenatal cell-free DNA screening might cause anxiety, it might help you avoid the need for more invasive tests, treatment or monitoring during your pregnancy.

Keep in mind, however, that prenatal cell-free DNA screening doesn't screen for all chromosomal or genetic conditions. A negative test result does not ensure an unaffected pregnancy.

How you prepare

If you're interested in prenatal cell-free DNA screening, talk to your health care provider about its availability. Also, consider checking to see if your health insurance covers the cost of prenatal cell-free DNA screening.

Before you undergo prenatal cell-free DNA screening, your health care provider or a genetic counselor will explain the possible results and what they might mean for you and your baby. Be sure to discuss any questions or concerns you have about the testing process.

What you can expect

Prenatal cell-free DNA screening can be done as early as week 10 of pregnancy.

During the procedure

During prenatal cell-free DNA screening, a maternal blood sample is taken and sent to a lab. The lab analyzes the maternal and fetal DNA in the blood sample. A higher than expected ratio of chromosome 21 sequences indicates, for example, the likely presence of trisomy 21 in the fetus. Trisomy 21 is the most common cause of Down syndrome.

Typically, test results are available within two weeks.

Results

The reporting of results varies depending on the lab. Results might be reported as positive or negative, as high risk or low risk for an abnormality, or as a probability.

Results follow-up:

• Chromosomal conditions. If the test results are normal, you might not need further testing. However, if results indicate an increased risk that the fetus has a chromosomal abnormality, amniocentesis or CVS will be offered. Further testing using the sample from amniocentesis or CVS can be used to confirm the diagnosis.
• Rh factor. If you're Rh negative and the test results indicate that the fetus is Rh negative, you won't need medication to prevent your body from producing Rh antibodies during your pregnancy. If the test results indicate that the fetus is Rh positive and you've previously become sensitized, your baby will be carefully monitored.

Chromosomal abnormalities can't be corrected. If your baby is diagnosed with a chromosomal abnormality, you'll need to decide whether to continue with your pregnancy or what steps to take to care for your baby during pregnancy and after he or she is born. Your health care provider or a genetic counselor can help answer any questions you might have.

Regardless of the results of prenatal cell-free DNA screening, if you have risk factors that suggest your baby might be at increased risk of a chromosomal or genetic condition, you have the option of undergoing amniocentesis or CVS.