Chorionic villus sampling

Chorionic villus sampling (CVS) is a prenatal test used to detect certain genetic defects early in a pregnancy.

During the procedure, a small sample of the chorionic villi (small projections of placental tissue that attach the membrane of the embryo to the uterine wall) is taken for microscopic evaluation. This tissue is useful in detecting genetic defects because its cells have the same genetic makeup as the baby's cells.


Purpose
Chorionic villus sampling is used to detect up to 200 genetic defects, including disorders such as:
sickle cell anemia (a chronic form of anemia -- a condition marked by a decreased number of red blood cells or hemoglobin, a protein in red blood cells that contains iron -- characterized by crescent-shaped red blood cells)
most forms of cystic fibrosis (a disorder that causes the body to produce a thick, sticky mucus that plugs the lungs)
Down syndrome (a combination of defects that lead to some degree of mental retardation and characteristic facial features)

Indications
Chorionic villus sampling is typically done during the first trimester, between the 10th and 12th week of pregnancy. Your doctor may recommend CVS if:
You're age 35 or older.
You have a biological child with a genetic defect or a genetic defect was detected in a previous pregnancy.
You or your partner have or could be carriers of a genetic disorder.
You have a family history of a genetic disorder.
You've had several miscarriages.

Patient preparation
It's best if your bladder is full during the procedure. You'll be asked not to urinate for several hours before the test, and you may need to drink one to two glasses of water just before the test.

Procedure
The doctor will use ultrasound (sound waves that create an image of internal body parts) to guide the removal of the tissue sample. Depending on your anatomy, the sample may be taken through a needle inserted into your abdomen or a tube-like instrument inserted into your vagina and cervix. You may experience slight cramping during the procedure or feel a pinch when the sample is taken.
The procedure will take about 30 minutes. Afterward, the doctor will check your baby's heartbeat.


Postprocedure care
For at least several hours after the procedure, you may need to decrease your activity level and avoid heavy lifting or prolonged standing. Although some vaginal bleeding or cramping is common, call your doctor if you experience heavy bleeding, severe cramping, fever or foul-smelling vaginal discharge.
Initial results are often available within a few hours, and the full results usually arrive within 10 days. To confirm the diagnosis of a disorder, your doctor provider may recommend an amniocentesis (withdrawing a sample of amniotic fluid -- the fluid that surrounds and protects the baby in the womb -- for examination).

You may also be scheduled for an ultrasound within the next few days to check on your baby's well-being. If you have Rh-negative blood and your baby may have Rh-positive blood, you'll be given an injection of Rho immune globulin to help prevent Rh disease.


Benefits
Since CVS can be done during the first trimester, certain birth defects can be detected earlier in the pregnancy than with an amniocentesis. This is especially beneficial if you're considering ending the pregnancy if a significant defect is detected. A therapeutic abortion done during the first trimester after CVS is safer than one done during the second trimester.

Risks
Although few infants have been affected, there's some concern that early chorionic villus sampling may increase the risk of limb deformities, such as shortened or missing fingers and toes. The risk is estimated to be one infant in 3,000 CVS procedures. To reduce the risk, CVS isn't recommended before the 10th week after the last menstrual cycle.
If you have a normally positioned uterus, the risk of miscarriage during CVS is about one in 100. The risk increases to about five in 100 if your uterus is retroverted (flexed backward) and the procedure is done through your vagina and cervix, rather than your abdomen. There's also a slight risk of infection or bleeding.


Considerations
Chorionic villus sampling only evaluates the chromosomes. A negative test result doesn't guarantee the absence of a birth defect. For example, neural tube defects, fetal lung maturity, complications of Rh disease, or defects caused by a drug or developmental abnormality can't be identified through chorionic villus sampling. CVS can be done earlier in a pregnancy than amniocentesis, but the risks are slightly higher.
Discuss any concerns you may have about testing with your doctor.