Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS), also known as chorionic villus biopsy, is a prenatal diagnostic test where tissue samples from the placenta are tested for a chromosomal conditions, like Down's syndrome and Edward syndrome as well as genetic conditions, like cystic fibrosis and sickle cell anemia. It is performed between the 10th to 12th weeks of pregnancy.
The tissue sample taken from the placenta is called chorionic villi. Chorionic villi are finger-like extensions of placental tissue that contain the same genetic material as the fetus. This means that one can examine the genetic material of the fetus by taking samples from chorionic villi.
When is CVS offered?
CVS is not routinely offered in every pregnancy. It is offered when;
antenatal screening tests suggest a chromosomal abnormality, such as Down’s syndrome
there is a previous pregnancy with a genetic or chromosomal defect
there is a family history, meaning parents have or are a carrier of a known genetic disorder, such as thalassaemia, Tay-Sachs or muscular dystrophy
eggs are fertilized or pregnancy has occurred naturally after maternal age, which is 35.
It should be noted that CVS cannot detect certain birth defects, including neural tube anomalies. Alternative diagnostic methods, such as genetic amniocentesis or an ultrasound, may be suggested if neural tube anomalies are of concern.
How is CVS performed?
There are two methods to perform CVS testing: transabdominal and transcervical. Both will be guided through ultrasound imaging throughout the procedure.
During the transabdominal CVS procedure, a chorionic villi sample is taken through the abdominal region with the help of a needle. After your abdominal region is sterilized, a long thin needle will be inserted through your abdomen to reach the chorionic villi in the uterus. The sample is then withdrawn to the syringe, which may cause a slight discomfort.
During the transcervical CVS procedure, a chorionic villi sample is taken through the cervix with the help of a speculum and catheter. After your cervix is sterilized, a speculum will be inserted into your cervix to open your vagina to place the catheter for chorionic villi sample collection. Catheter reaches the placenta through the speculum, and the tissue sample is withdrawn via suction.
CVS can be performed as early as 10th week of pregnancy. Although it is not common after the 14th week, it can be considered if necessary. The procedure generally takes about 10 to 30 minutes. After the procedure, you may experience abdominal cramps close to menstrual pain for approximately 3 to 4 hours, and vaginal bleeding for a short period of time.
Results of CVS
The results of CVS will tell you if your baby will be suffering from a genetic or chromosomal defect. The results may take up to 3 weeks to conclude, and it should be noted that CVS doesn’t test for neural tube defects, like spina bifida, which can be tested via another prenatal test called amniocentesis.
Risks of CVS
There are several risks associated with the chorionic villus sampling. These risks include:
Miscarriage: According to theNHS, 1 out of every 100 pregnancies ends up with miscarriage after CVS. However, it is not possible to say for sure whether the miscarriage is due to CVS. Most miscarriages that occur after CVS are about 3-5 days after the procedure.
Re-test order: NHS also states that 1 out of 100 samples may end up inconclusive after the test is done. This could be due to sample inadequacy, contamination, pre or post analytical errors, or there might be, simply, not enough cells in the sample to get a result. In that case, your doctor will order the test again, but you can also discuss your options with your doctor.
Uterine infection: Infection is a possibility for every surgical procedure, but it is quite rare to get an infection because of CVS.
Rh sensitization: CVS could result in some of the baby's blood cells being introduced into your bloodstream. And if you’re Rh- and your baby is Rh+, you will develop antibodies against your baby’s blood, which will eventually attack the red blood cells of the baby.
If this is the case, you will receive an injection of a blood product known as Rh immune globulin after chorionic villus sampling. This will prevent your body from developing Rh antibodies, which if they are allowed to pass through the placenta, can cause damage to the baby.
If you already have an infection, like STI, or if you’re suffering from occasional vaginal bleeding during your pregnancy, you may not be advised to take CVS. These risks should be carefully considered with your doctor. The decision to be made between continuing the pregnancy with the possibility of a genetic or chromosomal defect in the fetus or taking the risks of a test to find out if this is the case is of great importance.
Alternatives of CVS: Amniocentesis
As previously described above, amniocentesis could be an alternative for CVS. However, it differs from CVSin terms of the time the test is performed and the extent of the test.
Amniocentesis is performed in between the 15th and 20th months of pregnancy, therefore you will have less time to consider the future of your pregnancy after you get your results. So, it is sometimes offered after your CVS fails.
Amniocentesis offers the diagnosis of neural tube defects, which is not included in the CVS panel.
There is also a risk for miscarriage in amniocentesis as is for CVS.
Prenatal genetic tests offer great information especially if you’re suspected of having a child with a chromosomal or genetic abnormality. However, it is a complicated process that needs to be carefully considered with your doctor. Your doctor will inform you with the best option suitable for your case.