Chorionic Villus Sampling (CVS)
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Chronic Villus Sampling
Chorionic Villus Sampling (CVS), also known as CVS, is a specialized, invasive prenatal diagnostic test performed early in pregnancy, typically between the 10th and 13th weeks of gestation. However, the primary purpose of CVS is to attain a small sample of tissue from the placenta, which is known as Chorionic Villi. However, this tissue is then sent to the genetics laboratory for a brief analysis. Because the placenta and the baby originate from the same fertilized egg, the chorionic villi share the same genetic and chromosomal makeup as the developing fetus. Also, this enables the CVS test to provide a definitive diagnosis for a wide range of chromosomal abnormalities, like Down syndrome, and the hundreds of particular single-gene genetic disorders, such as cystic fibrosis and Thalassemia.
Why is Chorionic Villus Sampling (CVS) Done?
CVS is an elective procedure, and it is offered to women who have an enhanced risk for a chromosomal or genetic condition in their pregnancy. Some of the common indications includes:
Advantage of CVS
However, the main advantage of CVS over other diagnostic tests, such as amniocentesis, is the timing. By offering conclusive answers in the first trimester, CVS allows parents to have important information about the baby's health much earlier in the pregnancy. This will provide early reassurance for those with the normal results or allow more time for the families to make a crucial decision and plan for the future if a diagnosis is confirmed. While the procedure carries a minor risk, it is a safe and highly accurate test in the hands of an experienced fetal medicine expert.
Understanding the Chorionic Villi and Genetic Diagnosis
For understanding how a CVS works, it is crucial to understand the development of the placenta and the genetic principles that will make the test accurate.
The Placenta and the Chorionic Villi
Just after conception, the fertilized egg divides and develops into a structure called a blastocyst. On one part of the blastocyst develops into the embryo, and the other part develops into the placenta. However, the placenta is a crucial organ that connects the developing baby to the uterine wall, offering nutrients and oxygen while removing waste products.
The chorionic villi are tiny, finger-like projections that are the fundamental functional units of the fetal part of the placenta. Also, they burrow into the uterine lining and establish the rich blood supply that is required to sustain the pregnancy.
The Critical Genetic Link
The scientific principle that underpins a CVS is that the Chorionic Villi are genetically identical to the fetus. Also, they originate from the same initial cell and therefore contain the same initial cell and contain the same DNA and the same 46 chromosomes. By taking a small sample of this placental tissue, doctors can make a whole genetic and chromosomal profile of the baby without needing to sample the baby directly, which is why a CVS is not a screening test that only assesses probability but a diagnostic test that will provide a definitive answer.
What Can a Chorionic Villus Sampling Diagnose?
A CVS can be used to find out the wide range of genetic conditions.
Chromosomal Abnormalities
These are some conditions that are caused by an error in the number or structure of the chromosomes. A CVS can accurately diagnose:
Single-Gene Genetic Disorders
These conditions are caused by a mutation in a particular gene. CVS can test for thousands of these disorders, but the laboratory should be looking for a particular disease based on the family's history or carrier screening results. Some of the common examples include:
What CVS Cannot Detect?
It is crucial to understand the limitations of the CVS test.
Neural Tube Defects: CVS cannot detect open neural tube defects such as Spina Bifida. However, the blood test for alpha-fetoprotein AFP which is also a part of the Quadruple marker screen, is done around 16-18 weeks and is a test used to screen for these conditions.
Structural Birth Defects: The test does not provide details about physical birth defects like heart problems or a cleft lip.
Also, these are assessed during the detailed mid-trimester ultrasound scan.
The CVS Procedure
A CVS is a delicate method performed by the fetal medicine expert under continuous ultrasound guidance to make sure the safety of the baby. Also, there are two ways to access the placenta and obtain the sample.
Transcervical CVS
It is the most common procedure.
Preparation: You will need to have a comfortably full bladder for the procedure, as this will help to position the uterus for better access.
The Procedure: You will have to lie on your back on an examination table, similar to a Pap test.
The doctor will clean your cervix and vagina with an antiseptic solution. Under continuous guidance from an ultrasound scanner on your abdomen, the doctor will pass a thin, flexible catheter through the vagina and cervix and into the placenta.
A syringe is then attached to the catheter, and a small sample of the chorionic villi is gently suctioned out. Also, you may feel some cramping, which will be similar to period pain, during this part of the procedure.
Transabdominal CVS
This process is utilized when the position of your placenta will make it easier to access through the abdomen.
Preparation: There is no special preparation needed, such as a full bladder.
The Procedure: You will lie on your back. The doctor will clean an area of your lower abdomen with an antiseptic and will then numb the spot with a local anesthetic injection. Under continuous ultrasound guidance, the doctor will insert a thin, hollow needle through your abdominal wall and uterus and into the placenta.
In both methods, the baby's heart rate is well checked before and after the method, which usually takes about 30 minutes.