Chorionic villus sampling is a method of fetal DNA sampling that can be used either to determine chromosomal abnormalities or the paternity of the child. This type of test is considered invasive and in much the same way as amniocentesis, this test carried certain risks which will be discussed further on. CVS can be carried out earlier than amniocentesis which makes this test the preferred method. It also differs in that it does not allow diagnosis of neural tube defects while amniocentesis does.
How is Chorionic Villus sampling carried out and why?
The test is essentially a chorionic villus biopsy. A biopsy is essentially the removal of a tissue sample, in this case a sample of chorionic villus (placental tissue) from the uterus. Chorionic villi are finger-like projections which line the placenta. They increase surface area and thus are able to absorb nutrients and oxygen in larger quantities and more efficiently. The biopsy can be one of two types:
- Transabdominal CVS – in this type of CVS biopsy, a catheter is inserted through the cervix and into the placenta in order to get a small sample of tissue.
- Transcervical CVS – The OBGYN guides a needle into the placental wall through the abdomen. An abdominal ultrasound will be carried out to determine the position of the fetus in order to decrease risk.
In this case, you will be required to drink lots of water before the ultrasound. You will also be asked to refrain from micturating. A full bladder will help make the ultrasound imaging clearer.
CVS is carried out to determine chromosomal defects in the fetus. Down syndrome, Hemoglobinopathies and Tay-Sachs disease are 3 of the hundred or so conditions that analysis of a Chorionic villus sample can reveal. Having this type of diagnostic prenatal test can help decide on the right prenatal and antenatal care, make changes in lifestyle and diet, begin looking for the right support groups or have an early termination of pregnancy. The sample collected can also be used for a prenatal paternity test. However, the sample collection is invasive and due to the risks, a non invasive prenatal paternity test would most likely be recommended. The test can be carried out anywhere between 10 and 13 weeks of pregnancy.
Will it be painful? What will I feel?
The test will not be painful although it can cause discomfort. Following the procedures you may experience a fever or chills. The transcervical procedure may result in slight vaginal bleeding. Overall the side- effects and the pain is very bearable but you need to consider the risks.
What are the risks of CVS?
The results of an analysis of chorionic villi for chromosomal abnormalities are pretty high, between 98-99%. Unfortunately, the test cannot in any way assess the severity of the defect and the implications on the life of the baby once it is born.
In around 1% of cases, CVS generates false positive results. In other words, the results will confirm the presence of a hereditary condition or genetic disorder when it is in fact, not present in the unborn fetus. If you are considering an abortion, you might want to keep the possibility of false positive results in mind. Other risks include the leakage of amniotic fluid which must be taken care of immediately. The baby’s life depends on the amniotic fluid which surrounds it.
CVS was also thought to cause damage to the fetus’s limbs. Studies have shown this risk to be far lower than originally believed. In fact, around 1 in every 6000 cases result in damage to the fetus’s limbs.
More risks include infection and rupture to the membranes of the body. If too much fluid is lost, the fetus may die. Further to this, 1 out of 100 Chorionic villus sampling procedures result in miscarriage. All side effects and risks of this invasive prenatal testing will be explained by the OBGYN.