Amniocentesis and Chrorionic Villus Sampling (CVS)
Amniocentesis and chorionic villus sampling (CVS) are prenatal diagnostic procedures used to detect whether a fetus has a chromosomal abnormality such as Down syndrome. They can also detect the presence of some inherited genetic disorders for those couples known to have an increased risk. Amniocentesis tests chromosomes in the fetal cells. CVS tests chromosomes in the placental cells.
During amniocentesis, a small amount of amniotic fluid (about 15 millilitres) is removed from inside the amniotic sac. Amniotic fluid is the water that surrounds the fetus. It is made by the kidneys of the fetus and contains fetal urine.
Chorionic Villus Sampling (CVS)
The chorion is the early placenta. Chorionic villi are small thread-like projections that make up part of the chorion and placenta. Cells of the chorionic villi almost always have the same chromosomes as the fetus. Therefore, abnormalities in the chromosomes of chorionic villi are also found in chromosomes of the fetus.
Common Reasons For Having An Amniocentesis Or CVS Test
- The result of a prenatal screening test may indicate that the fetus has an increased risk of an abnormality. The common screening tests are an ultrasound scan at 12 weeks, with or without a blood test at 10 to 12 weeks, or a blood test at 15 to 17 weeks.
- Some women choose to have the CVS test instead of a screening test. This is usually when they are older than about 35 to 37 years. The most common condition of concern is Down syndrome.
- If a woman has had a previous pregnancy with a chromosomal or inherited disorder, amniocentesis or CVS may be offered.
- Prenatal diagnosis may be offered when a parent has a family history of a genetic disorder or chromosomal abnormality that can be diagnosed prenatally.
- If an abnormality (that suggests a chromosomal abnormality) is found at the time of an ultrasound scan, amniocentesis or CVS is offered in most cases.
Limitations Of The Test
Advances in technology, science and genetics have helped the diagnosis of many, but not all, serious disorders. While amniocentesis and CVS tests do not look for every known birth defect, they are accurate methods of detecting most major chromosomal abnormalities.
Diagnostic Laboratory Tests
Cells collected by amniocentesis or CVS are tested for Down syndrome and other chromosomal abnormalities.
- Tests on amniotic fluid: Cells in the amniotic fluid are cultured (stimulated to grow and divide) before the chromosome test can be performed. The amniotic fluid may be tested for AFP; a raised level may be due to spina bifida.
- Tests on chorionic villi: Chorionic villi cells are cultured in the laboratory for 7 to 10 days until enough cells are available for the chromosome test. Complete results are usually available in about 10 to 14 days.
Possible Complications Of Amniocentesis And CVS
Amniocentesis and CVS are relatively safe procedures but do have risks. Despite the highest standards of practice, complications can occur.
The following possible complications are listed to inform and not to alarm you. There may be other complications that are not listed.
- Increased risk of miscarriage - the risk of miscarriage after amniocentesis is about one miscarriage in every 100 to 200 women tested.
- Infection of the uterus or fetus, or both; this is rare.
- Some research has suggested that development of arms, fingers, legs or toes may be disrupted if CVS is performed before nine weeks gestation; for this reason, a CVS procedure is done after 10 weeks gestation.