Current methods of fetal diagnostic testing involve obtaining samples of amniotic fluid (amniocentesis), placenta (chorionic villus sampling) or fetal blood or tissues. However, fetal blood or tissue is rarely used as it puts the fetus at risk of injury.
Due to this risk, development of rapid, safe, effective and non-invasive prenatal tests have been an area of research. Fetal genetic material can be found in maternal circulation as early as the first trimester and this discovery has been very promising in the abilities to screen for fetal disease.
Cell free fetal DNA testing, also known as noninvasive prenatal testing (NIPT) has been commercially available since 2011. These tests require a simple blood draw at 10 weeks gestation or later. Routine testing includes an assessment for Down’s syndrome (Trisomy 21) and two other genetic abnormalities considered to be more severe than Down’s syndrome called trisomy 18 and trisomy 13. Aneuploidy is the term used to describe the presence of an abnormal number of chromosomes in a cell. These tests can also provide an assessment of sex chromosomes/gender and some rare disorders associated with sex chromosomes such as Turners Syndrome. The results are usually available within two weeks and there is a 2-4 % chance that there will not be a sufficient amount of fetal genetic material resulting in a “no-call” result.
It is important to understand that these tests are considered “screening” tests or a risk assessment rather than a “diagnostic” test. Any abnormal result should be confirmed by a diagnostic test such as amniocentesis. The laboratories that initially developed noninvasive testing conducted their research in a population of women over the age of 35. Women age 35 and older are considered to be at increased risk for a chromosomal abnormality. The rationale for this was to improve the likelihood of detecting the condition and avoiding a false positive or a false negative test result. A false negative result is when the test is falsely interpreted to be negative when indeed the fetus is positive. A false positive test result is when the test is falsely interpreted as positive when indeed the fetus is negative. It is also very important to understand that these tests have not been studied as extensively in the lower risk population (e.g., women under the age of 35). Conventional screening tests such as the first trimester (nuchal translucency) or quadruple screen are better validated in the low risk population.
Contact MetroPartners OBGYN
If you have questions about cell free fetal DNA testing, give MetroPartners OBGYN a call and schedule an appointment.