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. More specifically, these 2 tests are described in 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 results are nondiagnostic. 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.
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!
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!