If you’ve read the prequel to today’s post, you will notice that the number of prenatal screening options available to a patient far exceeds the number of diagnostic options. Today, we will review the two diagnostic tests available during pregnancy. While these two options have been available for decades they remain the gold standard in prenatal testing. What are your diagnostic testing options? They are Chorionic villus sampling (CVS) and Amniocentesis.
CVS has been around since the early 1980s. This test is typically performed between 10-13 weeks of gestation. CVS is performed by collecting a sample of cells from the placenta. Cells are collected from the placenta because the baby is far too small at this point in pregnancy and is still going through key points of fetal development. We can get information about the baby from the placenta, however, because the baby and the placenta develop from the same group of cells following conception.
CVS is performed either transvaginally or transabdominally. The method of approach depends on where the placenta has implanted in the uterus. Since this is considered an invasive procedure, a risk of miscarriage exists, and is quoted to be up to 1.0%.
There are many testing options that can be performed on a CVS sample. A karyotype, or a picture of the baby’s chromosomes, can definitively diagnose or rule out the vast majority of chromosome abnormalities, including Down syndrome, trisomy 18 and trisomy 13. An additional test, called a microarray, can identify missing or extra pieces of genetic material. If you are concerned that a pregnancy may be affected with a specific inheritable condition due to your carrier screening results, ultrasound findings or family history, targeted testing for a specific inheritable condition can also be performed on the sample collected from the CVS procedure.
Amniocentesis was first used by German doctors in the 1880s to relieve pressure on the fetus due to excess amniotic fluid. Since the 1950s, it has only been used for prenatal diagnosis. Amniocentesis is typically performed at 16-22 weeks of pregnancy. Instead of collecting a sample of cells from the placenta, the amniocentesis is performed by collecting a sample of the amniotic fluid surrounding the baby. In the amniotic fluid are cells shed from the baby’s skin and intestinal tract.
A benefit of amniocentesis over CVS is that the risk for miscarriage is typically quoted as less than or equal to 0.5%. However, recent data suggests that in experienced centers, the risk of the two procedures is comparable. Any testing option that a patient wants to pursue on the sample collected from a CVS could be performed on the amniotic fluid sample collected from the amniocentesis.
Therefore, the main difference between CVS and amniocentesis is the time during pregnancy in which it is performed. The advantage of CVS is receiving diagnostic information about fetal health earlier in pregnancy than with amniocentesis. In terms of accuracy, both tests are expected to provide accurate diagnostic answers. Of note, there is a 1% possibility of a CVS reporting confined placental mosaicism, in which the results do not properly reflect the genetic information of the baby. This occurs when genetic changes occur in the growing placenta cells, after the cells that become the actual baby have already split off from this other set of placental cells. While this happens a minority of the time, it is a factor to take into account when deciding for or against CVS.
Remember that diagnostic testing is not for everyone. This is one of many options available to a patient during pregnancy. Some individuals feel they want a yes or no as soon as possible regardless of a risk of miscarriage, and some people would like to avoid a test with a risk of miscarriage altogether. For the patient that wants to avoid a risk of miscarriage, the screening options previously discussed are great tools to determine if any true risk for a condition exists that a patient may want to investigate further. Then, if a risk seems large enough to outweigh the risk of miscarriage that comes with diagnostic testing, it may be an option you pursue. Or not! These are all options.
No one can tell you what is right or wrong for you. However, you can make the proper informed decision for yourself after collecting all the information you need. Again, if you would like more information, I would suggest scheduling a genetic counseling session with a counselor in your area, or talking with your OB/GYN. Once again, this is your friendly neighborhood genetic counselor, signing off.