UNDERSTANDING GENETIC TESTING, Andrei Rebarber, MD (Video)
To begin, let’s discuss what genetic disorders are. Genetic disorders are caused by a change in a person’s genes. Aneuploidy is a condition where there are missing or extra chromosomes. In a trisomy, there is an extra chromosome, while in a monosomy, a chromosome is missing. Inherited genetic disorders are caused by changes in genes called mutations, and in some cases, both of the parents must carry the same gene to have an impacted child.
Amniocentesis and chorionic villus sampling (CVS) are two invasive tests that are available to determine various genetic factors in the fetus. Traditionally, these tests were completed to determine the fetal chromosomes; however, there are more genetic tests that are available currently, depending on conditions that may exist prior to the pregnancy. For example, if the parents are recessive genetic carriers for certain conditions, genetic testing may be available for some of these conditions to identify the 25% of affected offspring. If they want to detect any kind of spontaneous new mutations unique to the current pregnancy, they can also perform these tests. There are also other noninvasive tests in the first trimester such as nuchal translucency and noninvasive prenatal testing (NIPT), but these are not diagnostic tests and are used only for screening for certain specific conditions. In the second-trimester quad screening, which tests AFP, hCG, Estriol, Inhibin-A and NIPT may also be performed. CVS testing and Amniocentesis testing are described in more detail below by Dr. Rebarber:
A chorionic villus sample, or CVS, is generally completed between 10 to 12 weeks gestation but no later than 13 weeks and 6 days. The advantage of this test is that it is done earlier and, often, the results return sooner than an amniocentesis, so the patient is at an earlier gestational age when they can review the findings. During the test (under continuous ultrasound guidance), a needle is inserted abdominally, or a catheter or tube is inserted vaginally, into the placenta to obtain a sample of the placental cells (villi), which are then sent to the lab. Final results usually come back within 7-10 days for a karyotype or chromosome analysis. There is about a 1% chance that somebody who does a CVS might still require an amniocentesis because the results are non-diagnostic. Complication rates or miscarriage rates are extremely low for this procedure if performed in skilled hands (<1%).
An amniocentesis is performed at around 16 to 18 weeks but can be performed anytime in the pregnancy thereafter. It involves a needle stuck into the amniotic sac transabdominally to obtain amniotic fluid. The procedure is performed under direct ultrasound guidance. Fluid is drawn out and sent to the lab; the lab takes out cells from the fluid, grows them in culture and, about 2-3 weeks later, you get the results of a karyotype or a chromosome analysis. Additionally, the amniotic fluid is sent for an alphafetoprotein level to determine the presence of various fetal abnormalities such as spina bifida. Recently, increased genetic diagnostic testing exists where specific analysis is performed that can look for various spontaneous errors in the DNA itself. This level of testing is usually determined on a case-by-case basis.
Individuals may not be good candidates for chorionic villous sampling or amniocentesis if they have an infection that can be transferred to their baby during the test such as HIV/AIDS, hepatitis B, or hepatitis C but this is controversial and should be assessed on a case-by-case basis with your medical provider.
How to Choose Between Prenatal Screening and Diagnostic testing?
Any pregnant patient regardless of age can choose to undergo diagnostic testing instead of, or in addition to, prenatal screening. The main benefit of diagnostic testing is that it can detect more genetic conditions such as those caused by an extra chromosome and other disorders where chromosomes are missing or damaged. Diagnostic tests can also help identify many inherited disorders as well as new mutations depending on the testing requested.
Things to Consider When Deciding Whether to Have Prenatal Genetic Testing
It is a patient’s decision whether they would like to have prenatal testing, prenatal screening, or no genetic evaluation. If you are deciding on whether you should have prenatal genetic testing, there are some things to consider. It may be beneficial to think about how you would use the results of prenatal screening tests in your pregnancy care. It is important to remember that a positive screening test only tells you that you are at higher risk of having a baby with a genetic disorder. A diagnostic test should be done if you are looking for a definitive result.
The benefit of knowing if their baby will be born with a genetic disorder before birth is that they have the time to learn more about the disorder and plan for any medical care the child may need.
Some parents would not like to know this information before the child is born, which is completely fine. There is no right or wrong choice.
To learn more about these tests, consider a genetic counseling appointment. Speak with your physician or schedule an appointment with MFM Associates/Carnegie Imaging for Women specialist. Contact our office today!
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Maternal Fetal Medicine blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!