Prenatal cell-free DNA (cfDNA) Screening
To the Point– Cell-free DNA (cfDNA) screening is an optional blood test that can be performed at 9-10 weeks of the pregnancy and beyond that can screen for certain genetic conditions in the pregnancy and tell you if the chance of these conditions is higher or lower. Cell-free DNA also goes by the name noninvasive prenatal screening (NIPS), noninvasive prenatal testing (NIPT), or you may familiar with some of the brand names, such as Harmony, Panorama, MaterniT21, Verifi, and InformaSeq (among others). All of these tests are forms of cfDNA screening. Some have referred to this test as the new gender test because in addition to screening for some genetic conditions, cfDNA screening can also predict whether the baby is a boy or a girl. Results from cfDNA screening are not yes or no, and positive or “high risk” results should be confirmed by a diagnostic test such as amniocentesis if more definitive information is desired.
Cell-free DNA (cfDNA) screening is an optional blood test that can be performed beginning at around 10 weeks of pregnancy and can screen for certain genetic conditions by analyzing placental DNA in mom’s blood (placental DNA usually has the same genetic make-up as that of the developing baby) to determine if there is an increased or decreased chance of certain genetic conditions.
Though this testing began as a screening for Down syndrome, over time the list of conditions being screened has grown. The effects of the conditions screened for vary tremendously from mild to severe. Some of the conditions being screened for are so mild that adults may not even be aware that they have the conditions (for example, individuals with 47,XXX, 47,XYY), however the effects of 47,XXX and 47,XXY vary from person to person. Learn more here.
Other conditions such as trisomy 18 and trisomy 13 are typically are associated with severe health and developmental concerns which usually lead to a significantly shortened lifespan. cfDNA screening can also predict the gender of the baby.
Besides the common genetic conditions involving an extra or missing chromosome, many labs are now offering testing for a handful of conditions that fall under the category called microdeletion syndromes (22q11 deletion, 5p deletion, 1p36 deletion, 15q deletions, etc.). Not all laboratories screen for the same conditions, so it is important to check with your healthcare provider to find out which conditions you are being offered screening for.
Most of the conditions these tests screen for do not run in families. The chance that a pregnancy is affected with a condition depends on a number of factors, including maternal age, ultrasound findings, and other prenatal screening results.
If results indicate a high or increased risk, a diagnostic test such as amniocentesis can then be performed, if the patient desires, to determine whether the baby really has the condition or not. If the results indicate a low or decreased risk, the chance the baby has the condition is small, but not zero. Keep in mind that false-positives and false-negatives do occur with cfDNA screening. The chance of a false-positive or a false-negative result depends on the condition and which cfDNA screening lab is used.
Because cfDNA screening only requires a blood draw on mom, we often don’t consider any risk attached to this type of testing. While that is generally true, it can be helpful to think about what these test results may mean for you. Most of the time the test will come back low risk and many women feel relief based on these results. However, if the test indicates an increased risk it may create worry and uncertainty about what to do next. For more information, see How to Decide below.
There is a chance of not obtaining results at all (a “no-call” result). No call results may occur due to insufficient levels of placental DNA in mom’s blood, or because of subtle variations in the DNA sequence that don’t allow the test to work properly. Many women will get a result if they have the cfDNA screening repeated with a second blood draw. Some studies have shown that women who have a no-call result may have an increased chance of a chromosome condition in their pregnancy4 and for that reason, your healthcare provider may recommend genetic counseling or offer you the option of having diagnostic testing after receiving a no-call result rather than repeating the cfDNA screening blood work or after a repeat if it is still a no-call.
Most of the research studies regarding cfDNA screening have been performed primarily with a population of women who have a higher chance of a chromosome condition such as women who are 35 and older. There is some research that suggests benefits to previously available screening tests such as the Integrated Screen or Sequential screen for younger women (5).
Current recommendations from the American College of Obstetricians and Gynecologists (ACOG), the Society for Maternal-Fetal Medicine (SMFM) advise that healthcare providers should discuss the potential benefits and limitations of cfDNA screening as compared to other screening tests. These recommendations emphasize that patients should be supported to make informed decisions about prenatal testing including cfDNA that align with their values, interests, and goals.
How It Works
DNA is contained within nearly every cell of our body. Our cells are continuously dividing to create new cells. As cells break down, the DNA inside the cell is released into the blood as fragments or pieces of DNA. The “floating” DNA fragments present in the blood are known as cell free DNA. All pregnant women will have DNA fragments that are her DNA as well as DNA from the placenta. The placenta develops during pregnancy and provides oxygen and nutrients to the developing baby. The placenta usually has the same genetic information as that of the developing baby. However, in some cases, the genetic makeup of the placenta and that of the fetus (developing baby) are not the same. This can lead to false positive and false negative results.
There are a couple of different approaches to this testing, however, in general, the laboratory is looking for differences in the overall amount of genetic information in mom’s blood. For example, if the testing finds an increased amount of chromosome 21 material in the blood sample, there will be an increased chance that the baby has Trisomy 21 (Down syndrome).
To the Point- So, bottom line with results from cfDNA screening? They can potentially be confusing, especially when they come back abnormal, so be sure to ask your doctor or genetic counselor for clarification on any questions that you may have. And, cfDNA screening results are not definitive. Diagnostic testing such as amniocentesis is recommended to provide a more definitive answer about whether the baby does or does not have the condition. Additional things to keep in mind regarding cfDNA screening results include the following….
As discussed above, cfDNA is a screening test so results are not yes or no answers. For most labs, you will get a separate result for each of the conditions that they are screening for. For example, you may get a “positive/high risk “result for Down syndrome but normal results for Trisomy 13, Trisomy 18, etc. Each lab does vary in the wording of their results. Here are some examples:
1) Positive ( increased chance for a chromosome condition) or Negative (decreased chance for a chromosome condition)
2) Aneuploidy detected or no aneuploidy detected (aneuploidy= extra or missing chromosome material detected), No aneuploidy detected= no extra or missing chromosome material detected)
3) High risk/Intermediate risk/low risk
It is important to remember that extra or missing chromosome material may be detected in the mother’s blood due to reasons other than a chromosome condition in the baby such as differences between the placenta and baby’s genetic makeup or subtle variations in the DNA that causes an unusual reading of the test.
Again, if your result for one of the chromosome abnormalities returns positive/aneuploidy detected/high-risk, this does not mean the baby has the condition; it means there is an increased chance that the baby has the condition. For more detailed information it will be very helpful to talk to your healthcare provider, such as a genetic counselor.
The statistical term that answers this question for a population just like you (same age or same background risk) is called Positive Predictive Value. PPV is calculated based on knowing the sensitivity, specificity and background risk of the condition. The lower your background risk is for a certain condition, the lower the PPV will be.
Here’s an example- during pregnancy the chance that a 20 year old is carrying a baby with Down syndrome is about 1 in 1,000 whereas the chance for a 40 year old woman is about 1 in 100. The chance that a cfDNA screening test returning positive for a 20 year old woman is a true positive (baby actually has Down syndrome) may be about 50% with some cfDNA screening tests, whereas for the 40 year old woman, the chance her positive result is a true positive (baby actually has Down syndrome) is actually closer to 90%.
Talk to your provider about what the results of your cfDNA screening really mean.
Of note, some laboratories are quoting PPVs that are very high and there is not literature to back up their claims. See this blog for more information on this topic or talk with your provider, such as a genetic counselor.
Labs may or may not be reporting this statistic that answers this question, which is called the Negative Predictive Value (NPV). The NPV for cfDNA screening is typically very high. The likelihood that any of these conditions is present in a pregnancy is low and most test results will come back indicating a low-risk. However, false negatives do occur with cfDNA screening in some cases.
Since cfDNA screening is a screening test, your provider should offer you a diagnostic test, such as amniocentesis to confirm a positive cfDNA screening result if you desire definitive results. If you have chosen to do cfDNA screening for purposes of preparing for the delivery and what to expect, you do not necessarily need to have an amniocentesis; the choice is yours to make, but you should be aware that both false positive and false negatives occur with cfDNA screening. Decisions regarding whether or not to continue the pregnancy should not be made based on cfDNA screening results alone.
Many people feel reassured by their negative cfDNA screening results and do not desire any further testing. Some individuals want more thorough and definitive information regarding the chromosomes and may opt to do diagnostic testing such as, amniocentesis. False negative results do occur with cfDNA screening, and cfDNA screening evaluates for a more limited number of conditions than diagnostic testing.
How to Decide
The following are some questions and thoughts to consider that may be helpful as you decide whether or not cfDNA screening is right for you…
Ultimately, the decision to have cfDNA screening or any other prenatal screening or diagnostic test is yours to make and should reflect your own personal beliefs, values, needs and personality.
1) Benn P Chapman AR, Erickson K, et al. Obstetricians and gynecologists’ practice and opinions of expanded carrier testing and noninvasive prenatal testing Article first published online: 16 DEC 2013 DOI: 10.1002/pd.4272
5) Norton ME, Baer RJ, Wapner RJ, Kuppermann M, Jelliffe-Pawlowski LL, Currier RJ. Cell-free DNA vs sequential screening for the detection of fetal chromosomal abnormalities. Am J Obstet Gynecol. 2015 Dec 18.