There are a variety of prenatal tests that can be done to detect any birth defects or genetic problems such as Down Syndrome. They are all done at various points throughout a pregnancy. Chorionic villus sampling (or "CVS" for short) is a prenatal test that can be done earlier than some, including an amniocentesis.
How is Chorionic Villus Sampling Administered?
This test is administered by inserting a small tube into the uterus through the cervix. This is usually guided by an ultrasound. The medical practitioner will position the tube between the uterine lining and the chorion (the fetal membrane that will be on the placental side). This can be a little uncomfortable or painful, but should not be excruciating.
A sample of the chorionic villi from the placenta is then extracted and taken to a lab to be analyzed for genetic problems. Since the chorionic villi have the same genes as the baby, it is possible to detect such defects through this sampling. CVS is usually done some time between the 10th and 13th week of pregnancy.
Who is Chorionic Villus Sampling Recommended For?
CVS is generally recommended for women who have had previous pregnancies resulting in a baby with birth defects. It is also recommended for women with a family history of birth defects. Doctors may also suggest undergoing CVS if an earlier prenatal screening test had results that were cause for concern. Since there is some risk associated with chorionic villus sampling, it is usually only administered to women who fall into these categories.
Complications That CVS Can and Cannot Identify
Chorionic villus sampling can identify the following birth defects:
- Chromosomal abnormalities, such as Down Syndrome and Turner Syndrome
- Genetic defects, such as sickle cell anemia and cystic fibrosis
CVS is unable to detect neural tube defects such as spina bifida. These are usually identified in second trimester prenatal tests.
Risks of CVS
There are some side-effects to CVS, which can range from mild to serious. These include:
- Cramping and vaginal bleeding - this usually happens because of the tube entering through the cervix;
- Rh sensitization - this happens when some of the baby's blood cells enter the mother's bloodstream. A production of antibodies against the baby's blood cells will result if the baby has Rh-positive blood and the mother has Rh-negative blood.
- Miscarriage - this happens in about 1% of chorionic villus sampling tests.
Possibility of Inaccuracy
With CVS, there is a one percent chance that a false positive result will be given. This means that a possible defect may be identified when in fact no defect actually exists.
In addition, a result called a "mosaicism" may occur, which represents a discrepancy between the chromosomal makeup of the cells in the placenta and the cells in the baby. some of the cells that were cultured from the placenta may contain abnormal chromosomes as well as normal chromosomes. If a CVS test detects a mosaicism, an amniocentesis may be required regardless of whether the baby has a birth defect or not.
For more pregnancy-related information, please visit: www.complete-pregnancy-guide.com
References:
1. Cherry, Sheldon H., MD, Understanding Pregnancy & Childbirth, 1992
2. Eisenberg et al, What to Expect When You're Expecting, 1991
3. Curtis, Glade B., Your Pregnancy Week by Week, 1997
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