chorionic villus sampling

Definition

Chorionic villus sampling (CVS), also known as chorionic villus biopsy, is a prenatal test that can detect genetic and chromosomal abnormalities of an unborn baby.

Description

Chorionic villus sampling has been in use since the 1980s. This prenatal testing procedure involves taking a sample of the chorion frondosum—that part of the chorionic membrane containing the villi—for laboratory analysis. The chorionic membrane is the outer sac which surrounds the developing fetus. Chorionic villi are microscopic, finger-like projections that emerge from the chorionic membrane and eventually form the placenta. The cells that make up the chorionic villi are of fetal origin so laboratory analysis can identify any genetic, chromosomal, or biochemical diseases of the fetus.

Chorionic villus sampling is best performed between 10 and 12 weeks of pregnancy. The procedure is performed either through the vagina and the cervix (transcervically) or through the abdomen (transabdominally) depending upon the preferences of the patient or the doctor. In some cases, the location of the placenta dictates which method the doctor uses. Both methods are equally safe and effective. Following the preparation time, both procedures take only about five minutes. Women undergoing chorionic villus sampling may experience no pain at all or feel cramping or pinching. Occasionally, a second sampling procedure must be performed if insufficient villus material was obtained.

For the transcervical procedure, the woman lies on an examining table on her back with her feet in stirrups. The woman's vaginal area is thoroughly cleansed with an antiseptic, a sterile speculum is inserted into her vagina and opened, and the cervix is cleansed with an antiseptic. Using ultrasound (a device which uses sound waves to visualize internal organs) as a guide, the doctor inserts a thin, plastic tube called a catheter through the cervix and into the uterus. The passage of the catheter through the cervix may cause cramping. The doctor carefully watches the image produced by the ultrasound and advances the catheter to the chorionic villi. By applying suction from the syringe attached to the other end of the catheter, a small sample of the chorionic villi are obtained. A cramping or pinching feeling may be felt as the sample is being taken. The catheter is then easily withdrawn.

For the transabdominal method, the woman lies on her back on an examining table. Ultrasound enables the doctor to locate the placenta. The specific area on the woman's abdomen is cleansed thoroughly with an antiseptic and a local anesthetic may be injected to numb the area. With ultrasound guidance, a long needle is inserted through the woman's abdominal wall, through the uterine wall and to the chorionic villi. The sample is obtained by applying suction from the syringe.

The chorionic villus sample is immediately placed into nutrient medium and sent to the laboratory. At the laboratory, the sample is examined under the microscope and any contaminating cells or material is carefully removed. The villi can be analyzed immediately, or incubated for a day or more to allow for cell division. The cells are stopped in the midst of cell division and spread onto a microscope slide. Cells with clearly separated chromosomes are photographed so that the type and number of chromosomes can be analyzed. Chromosomes are strings of DNA which have been tightly compressed. Humans have 23 pairs of chromosomes including the sex chromosomes. Rearrangements of the chromosomes or the presence of additional or fewer chromosomes can be identified by examination of the photograph. Down syndrome, for instance, is caused by an extra copy of chromosome 21. In addition to the chromosomal analysis, specialized tests can be performed as needed to look for specific diseases such as Tay-Sachs disease. Depending upon which tests are performed, results may be available as early as two days or up to eight days after the procedure.

Chorionic villus sampling costs between $1,200 and $1,800. Insurance coverage for this test may vary.

There are alternate procedures for diagnosing genetic and chromosomal disorders of the fetus. Amniocentesis is commonly used and involves inserting a needle through the pregnant woman's abdomen to obtain a sample of amniotic fluid. Amniocentesis is usually performed in the second trimester at approximately 16 weeks gestation and the laboratory analysis may take two to three weeks. The two advantages of chorionic villus sampling are that it is performed during the first trimester and the results are available in about one week. However, as of 1997, amniocentesis is being performed in the first trimester, but this is still very rare. The risk of miscarriage after amniocentesis is 0.5–1% (one to two women out of 200) which is lower than that for chorionic villus sampling (1–3%).

A noninvasive alternative is the maternal blood test called triple marker screening or multiple marker screening. A sample of the pregnant woman's blood is analyzed for three different markers: alphafetoprotein (AFP), human chorionic gonadotropin, and unconjugated estriol. The levels of these three markers in the mother's blood can identify unborn babies who are at risk for certain genetic or chromosomal defects. This is a screening test which determines the chance that the fetus has the defect, but it can not diagnose defects. A negative test result does not necessarily mean the unborn baby does not have a birth defect. For instance, this screening test can only predict 60–70% of the fetuses with Down syndrome. Pregnant women who have a positive triple marker screen are encouraged to undergo a diagnostic test, such as amniocentesis (by the time an AFP is done, it is too late to perform a CVS).

— Belinda Rowland, PhD

(1) (Concurrent Versions System) A version control system for Unix that was initially developed as a series of shell scripts in the mid-1980s. CVS maintains the changes between one source code version and another and stores all the changes in one file. It supports group collaboration by merging the files from each programmer.

(2) See computer vision syndrome.

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Cardiovascular system

Intervention: Chorionic villus sampling
Model of human embryo 1.3 mm. long. (Villi of chorion labeled at lower right.)
ICD-10 code:	16603-00
ICD-9 code:	75.33
Other codes:

Chorionic villus sampling (CVS) is a form of prenatal diagnosis to determine chromosomal or genetic disorders in the fetus. It entails getting a sample of the chorionic villus (placental tissue) and testing it. CVS can be carried out 10-13 weeks after the last period, earlier than amniocentesis (which is carried out as early as 14-16 weeks).

Contents 1 Indications 2 Risks 3 Limitations 4 See also 5 References 6 External links

Indications

Possible reasons for having a CVS can include:

• Mother's age of 35 years or greater
• Abnormal first trimester screen results
• Increased nuchal translucency or other abnormal ultrasound findings
• Family history of a chromosomal abnormality or other genetic disorder
• Parents are known carriers for a genetic disorder

Risks

CVS carries a higher risk of harming the fetus than amniocentesis (miscarriages occur in around 1 in 100 to 1 in 200 cases with CVS, versus around 1 in 1,600 with amniocentesis^[1]). Apart from a risk of miscarriage, there is a risk of infection and amniotic fluid leakage. The resulting amniotic fluid leak can develop into a condition known as oligohydramnios which is low amniotic fluid level. If the resulting oligohydramnios is not treated and the amniotic fluid continues to leak it can result in the baby developing hypoplastic lungs (underdeveloped lungs). Additionally, there is a risk of CVS causing digit-reduction defects in the fetus if performed before 11 weeks (0.07%-0.10%).^[2]

It is important after having a CVS that the OB/GYN follow the patient closely to ensure the patient does not develop infection.

Limitations

A small percentage (1-2%) of pregnancies will have confined placental mosaicism, where some but not all of the placental cells tested in the CVS will be abnormal, even though the pregnancy is unaffected.^[3] Cells from the mother can be mixed with the placental cells obtained from the CVS procedure. Occasionally if these maternal cells are not completely separated from the placental sample, this can lead to discrepancies with the results. This phenomenon is called Maternal Cell Contamination (MCC).^[3] CVS can not detect all birth defects. It's used for testing chromosomal abnormalities or other specific genetic disorders only if there is family history or other reason to test.

See also

• Amniocentesis
• Percutaneous umbilical cord blood sampling
• Prenatal diagnosis

References

1. ^ Amniocentesis Risk Overrated?
2. ^ US CDC MMWR Recommendations and Reports: Chorionic Villus Sampling and Amniocentesis: Recommendations for Prenatal Counseling, July 21, 1995 / 44(RR-9);1-12
3. ^ ^{a b} Wapner, Ronald (December 2005). "Invasive prenatal diagnostic techniques". Seminars in Perinatology 29 (6): 401–4. doi:10.1053/j.semperi.2006.01.003. 

External links

• March of Dimes
• MedlinePlus Encyclopedia 003406
• Cleveland Clinic
• CVS Test: Six Months of Worry Free Pregnancy

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