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Chorionic Villus Sampling

Chorionic Villus Sampling, or CVS, is a prenatal screening test used to detect chromosomal abnormalities, such as Down Syndrome, and inherited diseases, such as Tay-Sachs and hemophilia; however, it cannot detect neural tube defects such as spina bifida or anencephaly. Many women opt to have this test because it can be performed earlier than amniocentesis and the results are available much more quickly.

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CVS is generally performed between the 9th and 14th week of pregnancy and analyzes the genetic makeup of cells taken from the tiny finger-like projections on the placenta called chorionic villi. Depending on where your placenta is attached to your uterus, your doctor will perform either a transcervical or a transabdominal CVS. For a transcervical CVS, which is more common, your doctor will clean your vagina and cervix with an antiseptic and then thread a catheter through your cervix and, using an ultrasound to guide the catheter, will withdraw a sample of the chorionic villi. For a transabdominal CVS, your doctor will numb a spot on your abdomen and insert a long needle through your skin, muscle, and uterine wall to extract a sample. If your blood is Rh-negative, you will receive a shot of Rh immunoglobulin after the CVS (unless the baby's father is Rh-negative too) because it's possible that your baby's blood mixed with yours during the procedure.

The test takes only a few minutes and is relatively painless - most women compare it to a pap smear in discomfort. You'll need to take it easy after the test and abstain from strenuous physical activity, sex, and exercise for three days. You may experience some cramping and light bleeding afterward, which is normal; but if you notice any fluid leaking from your vagina or you have a fever, call your doctor immediately.

The cells extracted during the CVS are allowed to reproduce for a week or two and are then tested for chromosomal abnormalities. Results are available in seven to ten days, although some labs can give you preliminary results in two days. CVS can detect Down Syndrome and trisomy 18 with 98 to 99 percent accuracy.

There is a risk of miscarriage associated with CVS; however, it is very low - between one in 100 and one in 200. The risk is only slightly higher than that of amniocentesis, mostly due to the naturally higher risk of miscarrying in the first trimester. Amniocentesis can detect neural tube defects and some of the same inherited genetic disorders detected by CVS and can also help your doctor estimate your baby's lung maturity in the third trimester; however, an it cannot be performed until after the 14th week.

You should speak with a genetic counselor to help you decide whether CVS is appropriate for you. Due to its invasive nature and the risk of complications, you may want to have a noninvasive screening test, such as the nuchal translucency test or the first trimester combined screening, before you decide to have CVS.


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