Chorionic villus sampling (CVS) is a prenatal test carried out during pregnancy to detect any chromosomal or genetic abnormality in an unborn baby.
In this procedure, a small sample of chorionic villus is removed from the mother’s body and tested for the presence of any abnormality in the baby.
Chorionic villus is the tissue present in the placenta (the organ which connects the blood supply of the mother with her unborn baby).
Though the test procedure carries some risk, the benefits may outweigh them if there is a possibility that the baby may have an abnormal condition.
Not every pregnant woman requires a CVS test.
It is only required if there is a high risk of the baby having a genetic or chromosomal disorder. This includes
- Abnormal first-trimester screening test
- Increased nuchal translucency or other abnormal ultrasound findings
- History of a chromosomal or genetic disorder in a previous pregnancy
- Family history of a chromosomal or genetic disorder
- Parents are known to be carriers of a genetic disorder
- Advanced maternal age (> 35 years)
Read more about Level I and Level II ultrasound in Pregnancy
When is CVS Done?
This test is usually done between 10 and 13 weeks of pregnancy. In special circumstances, the CVS might be carried out later.
It should not be done before 10 weeks of pregnancy since there is a higher risk of complications like miscarriage or other birth defects induced by the procedure. Rarely, defects in baby’s fingers or toes have been reported, if CVS is performed before 10 weeks.
Diseases Identified by Chorionic Villus Sampling
CVS can detect a large number of chromosomal and genetic diseases. Some of the common ones include the following:
- Down syndrome
- Edwards syndrome
- Patau’s syndrome
- Cystic fibrosis
- Tay-Sachs disease
- Sickle cell anemia
- Sex-linked disorders like Duchenne muscular dystrophy (that occur most often in males)
- The test can also be used for paternity testing prior to the birth of the baby. DNA extracted from tissue sample obtained during the procedure is compared to DNA obtained from the potential father.
Read more about Genetic Inheritance: Modes and Significance
Some parents may choose not to undergo CVS testing for the following reasons:
- Based on moral or religious reasons, aborting the pregnancy is not an option.
- They do not want to undergo any testing that can harm the developing baby
Limitations of the Procedure
- Chorionic villus sampling does not test for every disorder that can affect a baby.
- Although the CVS test results are highly accurate, it may be difficult to identify some genetic abnormalities.
- Confined placental mosaicism: It is a condition in which there is a discrepancy between the chromosomal makeup of the cells in the placenta and the baby’s cells. Thus, although some placental cells tested in CVS are found to be abnormal, the baby is normal. This is seen in about 1-2% of pregnancies.
- Maternal Cell Contamination: During the procedure, cells from the mother can get mixed with the placental cells. In case, these maternal cells are not separated from the placental tissue, it can cause discrepancies in the results.
CVS cannot detect the following disorders
- Neural tube defects like spina bifida.
- Birth defects like cleft lip or cleft palate
- Rh incompatibility
- Disorders related to brain function, like autism, intellectual disability, etc
CVS is not recommended in the following situations:
- Vaginal infection
- Twin pregnancy
- Presence of uterine fibroids
- Presence of tilted uterus
- History of vaginal bleeding during pregnancy
Difference between Amniocentesis and Chorionic Villus Sampling
Amniocentesis is a procedure in which a small sample of amniotic fluid, (the fluid that surrounds the baby in the uterus), is removed for testing. It is usually carried out at about 15-20 weeks of pregnancy. Similar to amniocentesis, it is also used to detect genetic and chromosomal disorders. In addition, amniocentesis can detect neural tube defects. It also has several other applications like diagnosis of fetal infections, Rh sensitization, stem cell harvesting and treatment of polyhydramnios.
Read more about Amniocentesis- Indications, Procedure, and Complications
Preparation for Chorionic villus Sampling Test
No dietary or fluid restrictions are required before the test. Any medicines that have been advised should be taken routinely as prescribed by the doctor.
The patient will be asked to drink one to two glasses of water or any other fluid before the test and refrain from urinating. This is recommended because it helps the bladder to become full which enables better visualization on ultrasound.
Before performing the test, the couple must be provided with appropriate genetic counseling. This should include a detailed discussion regarding the benefits, risks, complications and the exact procedure.
The parents should weigh all the pros and cons of undergoing the test before giving their consent.
An ultrasound exam is performed to confirm the gestational age of the baby and to look for the location of the placenta. The mother is made to lie on her back with the abdomen exposed. A special gel is applied on the abdomen. The ultrasound image is used as a guide and the chorionic villus sample is taken from the placenta while the mother lies still.
There are two ways to collect chorionic villi from the placenta: through the abdomen or through the cervix.
Transabdominal chorionic villus sampling.
The skin over the abdomen is cleaned with an antiseptic lotion. A long, thin needle is inserted through the abdominal wall and then into the uterus. The tissue sample from the placenta is withdrawn into a syringe, and the needle is removed. This procedure is similar to amniocentesis.
Transcervical chorionic villus sampling.
The external genitalia and vagina are cleaned with an antiseptic.
A speculum is inserted into the vagina and opened to expose the cervix (the procedure is similar to a pap test). A thin hollow tube is inserted through the cervix. Under ultrasound guidance, the tube is guided up to the placenta. Gentle suction is then used to remove a small tissue sample.
The sample of chorionic villi, thus obtained is then sent to a genetics lab, where the cells are grown in a special medium and analyzed by FISH or PCR. The test results take about two weeks.
Risks and Complications of Procedure
Bleeding or cramping
There may be cramping or bleeding after CVS. It occurs more commonly if the procedure is performed through the cervix instead of the abdomen.
Leakage of amniotic fluid
Amniotic fluid leakage can develop into a condition known as oligohydramnios, which means a low level of amniotic fluid. If left untreated and the amniotic fluid continues to leak, it can result in underdeveloped lungs in the baby.
There is a small risk of infection similar to any invasive procedure. However serious infection is very rare.
CSV carries a small risk of miscarriage which is less than 1 in 100. The risk is more if the procedure is performed through the cervix instead of the abdomen. The risk also increases if the fetus is small for gestational age.
The risk of miscarriage is slightly greater than that for amniocentesis because CVS is done in early pregnancy.
Fetal limb deformities
Deformities of the baby’s limbs, commonly the fingers and toes can be observed, especially if the procedure is performed before ten weeks of pregnancy.
There is a possibility that during the procedure, the baby’s blood may get mixed with the mother’s blood.
This becomes significant in case the mother is Rh-negative and the baby is Rh-positive. When the fetal blood cells enter the mother’s body, the mother’s body develops antibodies against Rh factor present on fetal RBCs; that can attack the baby’s blood cells.
If this occurs, the mother should be given a medication called Rh immune globulin which will prevent the mother from becoming sensitized.
Read more about Rh Immune Globulin
Read more about Fetal Maternal Haemorrhage
After the procedure, the vital signs are checked and the patient is examined for any bleeding.
In case the mother is Rh-negative, she may be given Rh immunoglobulin to prevent Rh-negative mother’s antibodies from reacting with Rh-positive fetal cells
The patient is advised to rest for about a day or so after the procedure. Any strenuous physical activity, sexual intercourse, or exercise should be avoided for at least three days after the procedure.
Some amount of pain or bleeding is normal, but in case of excessive bleeding or leakage of fluid from the vagina, the health care provider should be informed immediately. Fever and severe abdominal cramps should also be reported.
A repeat ultrasound is scheduled about 2 to 4 days after the procedure to make sure that pregnancy is proceeding normally.
Interpretation of the Result and Follow-up
It means that there are no signs of genetic defects in the baby.
However, it is important to keep in mind the above-mentioned limitations of the test.
Chorionic villus sampling can detect a large number of genetic disorders. Abnormal results can occur in the presence of any genetic or chromosomal disorder.
The test result should be discussed with the treating doctor or genetic counselor who can explain the meaning of a specific test result. The following aspects should be discussed.
- Understanding the disorder affecting the baby
- Whether the disorder is treatable either during pregnancy or after the birth of the baby
- If pregnancy is continued, what abnormalities would the child have or what would be the special needs of the baby
The following options are available in case of an abnormal result
- Continuing with the pregnancy. The parents may opt to gather all the information about the condition so that they know how to care for the baby
- Terminating the pregnancy. This would also require emotional support and counseling.
In some cases, the CVS test may not give conclusive results. In such a situation, it may be necessary to have further tests, such as amniocentesis to confirm a diagnosis.
- CDC. Chorionic Villus Sampling Meeting. Federal Register 1994;59:8994.
- Kuliev AM, Modell B, Jackson L, et al. Risk evaluation of CVS. Prenat Diagn 1993;13:197-209.
- Verp MS. Prenatal diagnosis of genetic disorders. In: Gleicher N., ed. Principles and practice of medical therapy in pregnancy. 2nd ed. Norwalk, CT: Appleton and Lange, 1992:159-70.
- Jackson LG, Zachary JM, Fowler SE, et al. A randomized comparison of transcervical and transabdominal chorionic-villus sampling. N Engl J Med 1992;327:594-8.
- Lilford RJ. The rise and fall of chorionic villus sampling: midtrimester amniocentesis is usually preferable. Br Med J 1991;303:936-7.