Chorionic villus sampling

What it is

Like amniocentesis, chorionic villus sampling (CVS) can detect chromosomal and other genetic abnormalities in your unborn baby. But instead of sampling amniotic fluid, CVS examines tissue from the placenta. Because no amniotic fluid is collected, CVS can't test for neural tube defects, such as spina bifida.

Part of the placenta is a membrane layer called the chorion. Tiny, hair-like projections called villi extend out of the chorion and act as routes for nutrients, oxygen and antibodies from you to your baby. These chorionic villi contain fetal cells complete with your baby's DNA.

During a CVS procedure, your doctor takes a sample of chorionic villus cells from the placenta. This is done by inserting either a thin tube through

FISH: Speeding up genetic analysis

A method of genetic analysis that is faster than traditional techniques is now available. It's called fluorescence in situ hybridization (FISH), and it can deliver results in as little as 24 hours. In contrast, it will take from several days to two weeks to get results from a standard approach.

With amniocentesis, there aren't enough fetal cells collected in the sample to immediately perform a reliable comprehensive chromosomal analysis. Lab technicians must wait for sample cells to divide and multiply (culture). They then capture them at the phase of cell division when chromosomes can be seen as separate structures (metaphase). Once this occurs, the cells can be studied. The cell's chromosomes are then stained, revealing a unique pattern that allows them to be counted and analyzed under a fluorescent microscope.

FISH uses a technique that relies on short sequences of DNA called probes. These probes, which have a fluorescent tag attached to them, are designed to seek out and attach (hybridize) to specific genetic sequences in a cell sample.

Normally, a baby has two of every chromosome, totaling 23 pairs of chromosomes. Occasionally, a chromosomal abnormality occurs. This is the case with Down syndrome, which is characterized by three number 21 chromosomes. Abnormal numbers of chromosomes are called aneuploidies. Along with Down syndrome, other common aneuploidies that can be identified by FISH include:

• Extra or missing X (female) or Y (male) sex chromosomes

With FISH, for example, a red-labeled chromosome 21 probe will seek out all the number 21 chromosomes in a cell sample and mark them as red. When the sample is viewed under a fluorescent microscope, the red markers are easy to see and count. FISH analysis can be used on cells that aren't in the division process (interphase stage) so culturing isn't needed. This allows a much faster evaluation.

your cervix or a needle through your abdomen, guided by ultrasound views of the uterus. The sample is then sent to a laboratory for analysis.

When it's administered

CVS is usually performed between the ninth and 14th weeks of gestation. That's earlier in the pregnancy than amniocentesis is generally done. If you wish to have a diagnostic test early in your pregnancy, your doctor will probably recommend CVS over early amniocentesis because of the increased risk of miscarriage that early amniocentesis carries.

How it's done

A chorionic villus sample may be taken transcervically or transabdominally. Both approaches are considered equally safe. Which is used depends on the

The FISH test can determine the sex of the fetus and any numeric chromosomal problems that include the chromosomes X, Y, 13, 18 and 21. Unlike the reproductions here, actual FISH tests include colors that indicate the results.

Using FISH for rapid diagnosis often lets you know the results of your test much sooner than if conventional chromosomal analysis is used. That may give you more time to make decisions regarding your pregnancy.

Detection of common aneuploidies using FISH is generally very accurate. It has false-positive and false-negative rates of less than 1 percent.

But the technique has some limitations. Although accurate, FISH only identifies the problems listed above. And, although the number of chromosomes for 13, 18, 21, X and Y may be readily apparent during FISH analysis, the actual structure of the chromosome isn't apparent. Thus, structural abnormalities, which can indicate a problem, aren't evident with FISH, but they are with the conventional genetic analysis used with amniocentesis. In addition, FISH can't tell the difference between maternal cells and fetal cells. If maternal cells have contaminated the sample, the results may not truly reflect the genetic status of the baby. For this reason, most health care providers use FISH as a supplement to diagnosis, not as the sole basis for prenatal decisions.

position of the placenta and your doctor's experience. In general, placentas on the back side of the uterus are easier to sample through the cervix. Placentas in the front allow either approach.

An ultrasound is done before the procedure to determine the position of the placenta. Both variations of the procedure are done with ultrasound guidance.

• Transcervical CVS. This type of CVS procedure may feel similar to a Pap test with just slightly more cramping. You lie on your back with your feet in stirrups. After cleansing your vagina and cervix with antiseptic, your doctor inserts a thin, hollow tube (catheter) through your vagina and cervix into the chorionic villi. Gentle suction is then used to obtain a small placental cell sample. Some women have noticeable discomfort, and some don't.

• Transabdominal CVS. This procedure is similar to amniocentesis in that it uses a long, thin needle to obtain the cell sample. A safe entry site for the needle is determined using ultrasound, and your abdomen is

Ultrasound transducer

A vaginal speculum opens the vagina, and a catheter is inserted through the cervix into the chorionic villi during a transvesical chorionic villus sampling (CVS). A sample is gently removed by suction for testing in a laboratory As with amniocentesis, the doctor uses an ultrasound image to check the position of the fetus and guide the catheter into position.

Vaginal speculum

Chorionic villi

Chorionic villi

Vaginal speculum

Catheter

Catheter cleansed with antiseptic. Then your doctor inserts the needle through your abdomen into the chorionic villi and removes the sample.

The procedure takes about 45 minutes. The needle is in place only for a small part of that time. With CVS, generally a larger sample of cells is collected than with amniocentesis, so results of CVS are available sooner — within two to seven days, depending on the complexity of the laboratory analysis.

What the results may tell you

As in amniocentesis, analysis of fetal cells in the sample can reveal whether your baby has a chromosomal abnormality, such as Down syndrome, or another genetic disorder, such as Tay-Sachs disease, if there's reason to look for them.

Possible concerns

If you have a cervical infection, such as chlamydia or herpes, transcervical CVS isn't recommended. CVS requires more expertise than amniocentesis, so it's important to have an experienced obstetrician perform the procedure. In general, the risks of CVS are similar to those of amniocentesis:

• Miscarriage. The risk of miscarriage with CVS is slightly higher than with amniocentesis — about one in 100 (1 percent).

• Post-procedural complications. Vaginal bleeding occurs in about 7 percent to 10 percent of transcervical procedures, but rarely in transabdominal procedures. Cramping also may occur. Call your health care provider if you experience any of these problems. Infections are rare, but contact your health care provider if you develop a fever.

• Rh sensitization. As in amniocentesis, it's possible that some of your blood may mix with the baby's blood. If you're Rh negative, your health care provider will probably give you an injection of RhIg after the procedure to prevent you from producing antibodies against your baby's blood cells.

Several years ago, there was some controversy about whether CVS causes an increased incidence of limb defects. Reports at that time showed a slight increase (one in 3,000) in limb malformations with CVS. Since that time, other studies have detected no increase. Based on existing evidence, researchers have generally concluded that there's no association between these types of defects and CVS performed after 10 weeks of gestation.

Reasons to have it done

Both CVS and amniocentesis can provide genetic information about your baby. The advantage of CVS is that it's available earlier in pregnancy. CVS can also detect a few extremely rare genetic disorders in families with known risk that can't be found with amniocentesis.

What happens next

If the results are normal, no further testing may be required. If there's evidence of a chromosomal abnormality or other genetic disorder, you and your health care provider or genetic counselor can discuss the next step. In about 1 percent of cases, CVS results are unclear and amniocentesis may be necessary to confirm the diagnosis.

If ending the pregnancy is a consideration for you, the early availability of a CVS diagnosis may be an advantage. Ending the pregnancy sooner rather than later is generally safer and has fewer complications.

Early diagnosis can also be helpful in the treatment of certain disorders. For example, if a female baby has congenital adrenal hyperplasia, a condition in which excessive amounts of male hormones are produced, hormone therapy may be given to the mother to prevent the baby from developing male characteristics.

Accuracy and limitations of the test

CVS has less than a 1 percent chance of yielding false-positive results. A false-positive means that the test indicates the baby has an abnormality when there really is none. If you get negative results, you can be fairly certain that no chromosomal abnormalities are present in your baby. But CVS can't be used to check for all conditions. For example, it can't be used to test for neural tube defects, such as spina bifida.

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