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Chorionic Villus Sampling

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.

Purpose

Chorionic villus sampling is performed on pregnant women who are at risk for carrying a fetus with a genetic or chromosomal defect. Although it carries a slightly higher risk, CVS may be used in place of amniocentesis for women who have one or more of the following risk factors:

  • Women age 35 and older. The chance of having a child with Down syndrome increases with maternal age. For instance, the chance of having a baby with Down syndrome is one in 378 for a 35-year-old woman and increases to one in 30 for a 45-year-old woman.
  • A history of miscarriages or children born with birth defects.
  • A family history of genetic disease. Prenatal genetic testing is recommended if either the mother or father of the unborn baby has a family history of genetic disease or is known to be a carrier of a genetic disease.

Precautions

Chorionic villus sampling is not recommended for women who have vaginal bleeding or spotting during the pregnancy. It is not typically recommended for women who have Rh sensitization from a previous pregnancy.

Description

Chorionic villus sampling has been in use since the 1980s. This prenatal testing procedure involves taking a sample of the chorion frondosumthat part of the chorionic membrane containing the villifor 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.

Alternate procedures

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).

Preparation

Prior to the chorionic villus sampling procedure the woman needs to drink fluids and refrain from urinating to ensure her bladder is full. These preparations create a better ultrasound picture.

Aftercare

It is generally recommended that women undergoing chorionic villus sampling have someone drive them home and have no plans for the rest of the day. Women with Rh negative blood must receive a Rho (D) immune globulin injection following the procedure. Women should call their doctor if they experience excessive bleeding, vaginal discharge, fever, or abdominal pain after the procedure.

Risks

Of women who undergo transcervical chorionic villus sampling, one third experience minimal vaginal spotting and 7-10% experience vaginal bleeding. One out of five women experience cramping following the procedure. Two to three women out of 100 (or 2-3%) will miscarry following chorionic villus sampling. The risk of infection is very low. Rupture of the amniotic membranes is a rare complication. Women with Rh negative blood may be at an increased risk for developing Rh incompatibility following chorionic villus sampling.

KEY TERMS

Chorionic villi Microscopic, finger-like projections that emerge from the outer sac which surrounds the developing baby. Chorionic villi are of fetal origin and eventually form the placenta.

Chromosomes Human cells carry DNA in tightly compressed rod-like structures called chromosomes. Humans have 23 pairs of chromosomes including the sex chromosomes.

Down syndrome A chromosomal disorder caused by an extra copy or a rearrangement of chromosome 21. Children with Down syndrome have varying degrees of mental retardation and may have heart defects.

Fetus Term for an unborn baby after the eighth week of pregnancy. Prior to seven weeks, it is called an embryo.

Rh sensitization A woman with a negative blood type (Rh negative) who has produced antibodies against her fetus with a positive blood type (Rh positive). The mother's body considered the fetal blood cells a foreign object and mounted an immune attack on it.

Ultrasound A safe, painless procedure which uses sound waves to visualize internal organs. A wand that transmits and receives the sound waves is moved over the woman's abdomen and internal organs can be seen on a video screen.

There have been reports of limb defects in babies following chorionic villus sampling. However, in 1996 the World Health Organization reported that the incidence of babies born with limb defects from 138,966 women who had undergone chorionic villus sampling was the same as for women who had not. Therefore, this study found no connection between chorionic villus sampling and limb defects.

Normal results

No genetic, chromosomal, or biochemical abnormalities were found in the fetal cells. The gender of the fetus will be identified but will be made known to the parents only with their approval.

Abnormal results

Analysis of the cells from the chorionic villus enables the detection of over 200 diseases and disorders such as Down Syndrome, Tay-Sachs disease, and cystic fibrosis. Gross rearrangements of the chromosomes and chromosome additions or losses are detected.

Resources

ORGANIZATIONS

March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (914) 428-7100. [email protected] http://www.modimes.org.

OTHER

Family Internet Page. http://www.familyinternet.com.

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chorionic villus sampling

chorionic villus sampling (CVS) or chorionic villus biopsy (CVB) (kōr´ē-ŏn´Ĭk, kôr´–), diagnostic procedure in which a sample of chorionic villi from the developing placenta is removed from the uterus of a pregnant woman (see pregnancy) using a fine needle inserted through the abdomen or a thin plastic catheter inserted into the vagina and through the cervix. Chorionic villi are fingerlike projections of a membrane (the chorion) that surrounds the fetus. The villi develop from the fertilized ovum, or egg, and have a genetic composition similar to that of the fetus. Cells in the sample are grown in the laboratory and studied to detect the presence in the fetus of such genetic birth defects as Tay-Sachs disease and Down syndrome. The sex of the child can also be ascertained. Although CVS tests for the same range of abnormalities as amniocentesis, it is usually performed some weeks earlier (between the 8th and 12th weeks of pregnancy), and the results are available in a few days. It is recommended if the parents are carriers of certain genetic diseases, if there is a family history of genetic disorders, or if the woman is over age 35 (later pregnancies carrying with them a higher risk of chromosomal abnormality).

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chorionic villus sampling

chorionic villus sampling (CVS) n. a procedure for the prenatal diagnosis of chromosomal abnormalities (e.g. Down's syndrome) in which a sample of placental tissue (containing chorionic villi) is aspirated through the cervix or abdomen under ultrasound visualization. The cells so obtained are subjected to chromosomal and biochemical analysis. CVS is usually performed between the 10th and 13th weeks of gestation.

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CVS

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CVS

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CVS

CVS Med., Zoology cardiovascular system
• Med. chorionic villus sampling
• Council of Voluntary Service

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Chorionic Villus Sampling

Chorionic Villus Sampling

Definition

Chorionic villus sampling (CVS) is a prenatal procedure for the removal by needle of chorionic villi and culture and examination of the fetal cells obtained. The cells are used in tests for genetic and chromosomal abnormalities of a fetus as early as 10 to 12 weeks of gestation.

Purpose

Women who are at risk of carrying a fetus with a genetic or chromosomal defect may be counseled to have a prenatal screening test such as CVS or amniocentesis. CVS, which is performed at 10 to 12 weeks after a woman's last menstrual period, may be offered as an alternative to amniocentesis, the more commonly used test for prenatal diagnosis of genetic disorders, which is performed usually at 15 to 18 weeks. Earlier diagnosis of congenital defects is especially beneficial in cases where the parents desire to know the results as early in pregnancy as possible, for instance, when therapeutic abortion is being considered, as the risks associated with abortion increase with gestational age.

Prenatal screening can diagnose some genetic and virtually all chromosomal disorders and is advised for women who have one or more of the following risk factors:

  • Women age 35 and older. The chance of having a child with certain chromosomal birth defects increases with maternal age. The most common chromosomal disorder is Down syndrome, a combination of mental and physical abnormalities caused by the presence of an extra copy of chromosome 21. The occurrence of Down syndrome in children born to women in their 20s is approximately one in 1,250, but increases to one in 400 by age 35, and to one in 100 at age 40.
  • A child or previous pregnancy with a birth defect, or a history of miscarriages. A woman who has already had a child or pregnancy diagnosed with a genetic birth defect or chromosomal abnormality, or who has had multiple miscarriages, is at increased risk of having a child with a genetic disorder.
  • Determination of the sex of a fetus when the mother is known to be a carrier of a sex-linked genetic disease (for example, hemophilia A).
  • Other family history of genetic disease. Couples who do not have an affected child but who have family medical histories of genetic or chromosomal abnormalities or are known through genetic screening to be carriers of an inherited disease are at increased risk of having an affected child. Prenatal testing is offered only when the suspected condition can be diagnosed before birth.

Precautions

Chorionic villus sampling usually is not recommended for:

  • A woman whose pregnancy has progressed further than about 12 weeks, counted from the first day of the last menstrual period.
  • A woman who is experiencing bleeding or spotting.
  • In the presence of certain vaginal infections.
  • When there are uterine abnormalities, such as a bicornuate (double) uterus or uterine fibroids.
  • A woman who would not consider terminating a pregnancy where a genetic abnormality is detected.

The risk of miscarriage associated with CVS is between 1 in 200 and 1 in 100. Women who are concerned about this risk can be offered a noninvasive alternative. One such alternative is a maternal blood test called triple marker screening or multiple marker screening, which is performed no sooner than 15 weeks but no later than 20 weeks of pregnancy. A sample of the pregnant woman's blood is analyzed for three substances produced by the fetus and passed into the mother's blood: alpha fetoprotein (AFP), human chorionic gonadotropin (HCG), and unconjugated estriol (UE3). Elevated AFP may indicate the presence of anencephaly or spina bifida, and HCG is increased and AFP and UE3 are decreased in Down syndrome. This screening test, however, does not definitively diagnose a genetic defect and has a relatively high rate of false negatives, but it can help to identify an unborn baby at increased risk for these birth defects. A pregnant woman whose triple marker results are indicative of a fetus at risk would be encouraged to undergo further screening, such as ultrasound and/or amniocentesis (the timing of the triple marker screening rules out CVS) to provide a definitive diagnosis.

One drawback of CVS is tha since only cells and not fluid are collected, CVS does not allow evaluation of AFP to indicate the presence of neural tube defects. AFP is then generally measured by a maternal blood test at 15-20 weeks, but with less accuracy than would be obtained with an amniotic fluid sample.

A CVS sample may be obtained either via the vagina and cervix (transcervical CVS) or through the abdomen (transabdominal CVS). Woman who have a retroverted (tipped) uterus should be sampled transabdominally, since studies have shown the risk of miscarriage to be lower than when the procedure is done transcervically. In cases where the location of the placenta contraindicates transabdominal CVS, amniocentesis should be offered as an alternative screening test. Amniocentesis, however, is only rarely performed in the first trimester.

Although CVS is over 99% accurate in ruling out certain chromosomal birth defects and specific genetic problems, it is slightly more likely than amniocentesis to give inconclusive results. Amniocentesis, then, may be used as a back-up screening procedure. However, the patient should be cautioned that not all birth defects can be ruled out before birth and no prenatal test can guarantee the birth of a healthy baby. Because it is performed earlier in pregnancy than amniocentesis, CVS sampling is more likely to detect fatal genetic diseases that result in miscarriages after the diagnosis has been established.

Description

Chorionic villus sampling has been in use increasingly since the 1980s. The CVS procedure involves taking a sample for laboratory analysis of the chorion frondosum—the part of the chorionic membrane that contains the villi, microscopic, finger-like projections that emerge from the chorionic membrane to form the placenta. The chorion is the outermost membrane surrounding the developing fetus; the amnion is the inner membrane that contains the amniotic fluid. The cells that make up the chorionic villi are of fetal origin, and, thus, normally have the same genetic makeup as the fetus. The sample may be obtained with a catheter, a thin, plastic tube, inserted through the vagina and the cervix (transcervical CVS) or with a needle through the abdominal wall (transabdominal CVS); both sampling methods take about five minutes to perform (not including preparation time), are equally effective, and carry similar risks.

Chorionic villus sampling is best performed between 10 and 12 weeks of pregnancy. Abdominal ultrasound is used to determine the position of the uterus, the position of the placenta within the uterus, and the size of the amniotic sack. The woman assumes the lithotomy position (on her back with her feet in stirrups). In transcervical CVS, the vulva, vagina, and cervix are thoroughly cleansed with antiseptic; no anesthetic is required. A speculum is inserted into the vagina and opened, then, using ultrasound as a guide, the doctor inserts a catheter through the cervix and into the uterus. The catheter is carefully advanced to the chorionic villi and suction is applied with the syringe attached to the catheter to obtain a small sample of the villi. The catheter is then carefully withdrawn.

In transabdominal CVS, the appropriate area on the woman's abdomen is cleansed thoroughly with antiseptic and a local anesthetic may be injected to numb the area. With ultrasound guidance to strictly avoid the placenta, a long needle is inserted through the woman's abdominal wall, through the uterine wall and to the chorionic villi. Suction is applied with the syringe attached to the needle to obtain a small sample of the villi, and the needle is then carefully withdrawn.

Most women report that transcervical CVS feels similar to the procedure for a Pap smear. The passage of the catheter through the cervix may cause cramping, and some women experience cramping or pinching sensations when the sample is taken. There is generally little or no discomfort associated with the transabdominal procedure. Occasionally, when insufficient villus material is obtained, a second sampling procedure must be performed.

The chorionic villus sample is immediately placed into a sample dish containing nutrient medium for transport to the cytogenetics laboratory. At the laboratory, the sample is examined under the microscope so that any contaminating cells or material may be carefully removed. The villi can be analyzed immediately, or incubated for a day or more to give the cells time to undergo division. When the cells are in the midst of dividing, they are spread onto a slide and examined under a microscope. Cells that have clearly separated chromosomes are photographed to allow analysis of the type and number of chromosomes. The chromosomal images are collected in a report called a karyotype, which shows the number, shape, size, and arrangement of chromosomal pairs. For biochemical studies, deoxyribonucleic acid (DNA) is extracted from cultured chorionic villus cells. Depending upon which tests are performed, results may be available as early as two days or up to ten days after the procedure.

Preparation

Thorough pre-CVS counseling is strongly recommended to give the couple the opportunity to make informed decisions about prenatal diagnosis. The couple should be provided with literature about CVS and genetic screening options to read prior to the counseling conference, so that any questions or concerns they may have can be addressed at that time. Prior to the procedure, the woman will likely be asked to sign a consent form.

For the procedure itself, the woman will be instructed to drink fluids and refrain from urinating so that the bladder is partly filled; excessive and uncomfortable bladder filling is not necessary. A filled bladder can help to properly position the uterus and create a better ultrasound picture for guiding the CVS procedure.

Aftercare

After the sample is taken, the fetal heartbeat is checked by ultrasound before the woman leaves the examination room. In many cases the woman's blood pressure, pulse, and breathing, and the fetal heartbeat will be monitored for 30 minutes following the procedure. A follow-up ultrasound procedure may be scheduled two to four days after CVS to ascertain the health of the fetus.

Most physicians recommend that the woman have someone drive her home after the CVS procedure and that she limit strenuous activity for the remainder of the day. A women who experiences excessive bleeding, vaginal discharge, fever, or abdominal pain after the procedure should consult her doctor.

Women with Rh negative blood may receive a Rho (D) immune globulin (RhoGAM) injection to avoid Rh incompatibility.

Advances in prenatal treatment have made it possible to treat some genetic defects diagnosed by prenatal screening before birth. Congenital adrenal hyperplasia (CAH), for example, is an inherited birth defect in which an enzyme deficiency causes a female fetus to develop abnormal external genitalia by the 16th week of pregnancy. Prenatal diagnosis by CVS in pregnancies known to be at risk of CAH allows the fetus to be treated with hormones during the critical period of development from 10 to 16 weeks of gestation and can circumvent the need for surgery after birth. For conditions in which prenatal treatment is not available, prenatal diagnosis can allow parents the opportunity to discuss their options with genetic counselors or other health care providers, to plan the delivery, and to prepare emotionally for the birth.

Complications

Of women who undergo transcervical CVS, many experience minimal vaginal spotting and some experience vaginal bleeding. Some women also experience cramping following the procedure. Rupture of the amniotic membranes is a rare but serious complication that can lead to infection and/or miscarriage. A woman with Rh negative blood who is carrying an Rh positive fetus may be at an increased risk for developing Rh incompatibility following CVS and should be treated with Rho immune globulin.

There have been some reports of babies born with missing or shortened fingers or toes and abnormalities of the tongue and lower jaw when CVS had been performed before the tenth week of pregnancy. These reports raised concerns about the safety of CVS, although this type of limb defect is known to occur in approximately 1 out of every 1,700 babies. The risk of these kinds of birth defects are considered the same or minimal with CVS if it is preformed after the tenth week. Currently CVS is only recommended to be preformed after the tenth week, and these risks are not considered significant.

KEY TERMS

Alpha fetoprotein— A serum protein produced by the fetus during pregnancy and passed to the mother's blood, useful in the prenatal diagnosis of multiple births or birth defects.

Amniocentesis— A prenatal screening procedure in which a sample of amniotic fluid from the amniotic sac in the uterus of a pregnant woman is obtained by inserting a needle through the abdominal wall.

Chromosomes— Tightly compressed rod-like structures that carry DNA; humans have 23 pairs of chromosomes including the sex chromosomes.

Cystic fibrosis— An inherited chronic disease of the exocrine glands, characterized by the production of viscous mucus that obstructs the pancreatic ducts and bronchi, leading to infection and fibrosis.

Down syndrome— Also called trisomy 21, a genetic disorder, associated with the presence of an extra copy or a rearrangement of chromosome 21, characterized by mild to severe mental retardation, weak muscle tone, a low nasal bridge, and epicanthic folds at the eyelids; formerly called mongolism.

Human chorionic gonadotropin— A hormone produced by the developing placenta that stimulates the production of estrogen and progesterone; its presence in maternal blood or urine is used to diagnose pregnancy.

Karyotype— A report in which photographic images of the chromosomes of a cell are displayed as a systematized arrangement of pairs in descending order of size.

Muscular dystrophy— An inherited disease in which muscles become gradually wasted and are replaced by scar tissue and fat, sometimes also affecting the heart.

Neural tube defect— Any of a group of inherited abnormalities of brain and spinal cord development, including spina bifida and anencephaly, caused by failure of the neural tube to close properly during development.

Rh incompatibility— An Rh negative person who is exposed to Rh positive red blood cells. When an Rh negative mother becomes pregnant with an Rh positive fetus, the fetal red blood cells may enter the maternal circulation and stimulate production of Rh antibodies. This process can be prevented by administration of Rho immune globulin prior to or during the pregnancy.

Sickle-cell anemia— An inherited chronic blood disease that occurs primarily among persons of African descent characterized by abnormal hemoglobin that causes red blood cells to become sickle-shaped and nonfunctional, leading to an enlarged spleen, chronic anemia, weakness, joint pain, and formation of blood clots.

Tay-Sachs disease A rare and fatal inherited disease occurring chiefly in persons of eastern European Jewish origin, characterized by a red spot on the retina, gradual blindness, and paralysis.

Ultrasound— Also called ultrasonography, a diagnostic imaging technique that uses reflected high-frequency sound waves to visualize internal body structures or organs, especially useful as a noninvasive prenatal diagnostic tool.

There is risk with CVS of getting a "placental mosaicism" artifact in which the cultured cells contain some abnormal chromosomes that originate in the placenta and are not related to the fetus. Mosaicism occurs when cells have two or more distinct chromosome counts. This occurs when nondisjunction (failure of chromatids to separate) occurs in germ line cells (after fertilization). The fetus may be normal, and the only way to rule out actual mosaicism is to follow up with amniocentesis. There is also a risk that insufficient chorionic villi are collected for analysis, or that the cells collected are contaminated with cells of maternal origin. In this case, a second sampling procedure is performed about a week later, or amniocentesis may be offered as an alternative.

Results

The chorionic villus cells are cultured and photographed through a microscope during cell division to obtain images of the chromosomes. The images are sorted, identified, and reported in a karyotype. Humans have 23 pairs of chromosomes, including the sex chromosomes. The karyotype allows detection of aneuploidies (extra copies of chromosomes), chromosomal deletions, and gross chromosomal translocations. The gender of the fetus is identified from the sex chromosomes, and can be reported to the parents upon request.

DNA studies are performed when indicated by a family history of genetic disorders. DNA is the biochemical molecule that stores genetic information in the chromosomes. The DNA is extracted from the chorionic villus cells, and analysis of the DNA allows prenatal diagnosis of over 200 diseases, including Tay-Sachs disease, cystic fibrosis, sickle-cell anemia, and muscular dystrophy.

Health care team roles

The genetic counselor, physician, or other health care provider will provide information to the couple considering CVS and explain the procedure and its risks, and alternative procedures. Counselors can also advise parents of their options when they learn that their unborn child has an inherited disorder and/or help them prepare emotionally for the delivery of a child with a birth defect.

The obstetrics team carries out the ultrasound and CVS procedure, and ensures that the chorionic villus cells are properly handled for transport to a cytogenetics laboratory. The team of physician(s) and nurses also provide practical and psychological support for the couple undergoing prenatal diagnosis.

Technologists in the cytogenetics laboratory perform cell culture on the chorionic villus cells, fix a sample of cells during cell division, prepare the cells on a slide for microscopic analysis, and make photographs of the chromosomes of dividing cells to be collated in a karyotype. Cytogenetic technologists are also involved in the extraction of DNA from the cells and the biochemical testing of the DNA.

Resources

BOOKS

Evans, Mark I. ed. Prenatal Diagnosis. New York: McGraw-Hill Medical Pub. Division, 2005.

PERIODICALS

Cunniff, Christopher. "Prenatal Screening and Diagnosis for Pediatricians." Pediatrics 114 (September 2004): 889-895.

"Prenatal Diagnosis: Amniocentesis and CVS." American Family Physician 78 (October 2005).

Walling, Anne D. "Earlier Amniocentesis and Chorionic Villus Sampling." American Family Physician 71 (March 2005): 1011.

ORGANIZATIONS

March of Dimes. 1275 Mamaroneck Avenue, White Plains, NY 10605. http://www.marchofdimes.org.

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Chorionic Villus Sampling

Chorionic villus sampling

Definition

Chorionic villus sampling (CVS) is a prenatal procedure for the removal by needle of chorionic villi and culture and examination of the fetal cells obtained. The cells are used in tests for genetic and chromosomal abnormalities of a fetus as early as 10 to 12 weeks of gestation.

Purpose

Women who are at risk of carrying a fetus with a genetic or chromosomal defect may be counseled to have a prenatal screening test such as CVS or amniocentesis . CVS, which is performed at 10 to 12 weeks after a woman's last menstrual period, may be offered as an alternative to amniocentesis, the more commonly used test for prenatal diagnosis of genetic disorders, which is performed usually at 15 to 18 weeks. Earlier diagnosis of congenital defects is especially beneficial in cases where the parents' desire to know the results as early in preg- nancy as possible, for instance, when therapeutic abortion is being considered, as the risks associated with abortion increase with gestational age.

Prenatal screening can diagnose some genetic and virtually all chromosomal disorders and is advised for women who have one or more of the following risk factors:

  • Women age 35 and older. The chance of having a child with certain chromosomal birth defects increases with maternal age. The most common chromosomal disorder is Down syndrome , a combination of mental and physical abnormalities caused by the presence of an extra copy of chromosome 21. The occurrence of Down syndrome in children born to women in their 20s is approximately 1 in 1,250, but increases to 1 in 400 by age 35, and to 1 in 100 at age 40.
  • A child or previous pregnancy with a birth defect, or a history of miscarriages. A woman who has already had a child or pregnancy diagnosed with a genetic birth defect or chromosomal abnormality, or who has had multiple miscarriages, is at increased risk of having a child with a genetic disorder.
  • Determination of the sex of a fetus when the mother is known to be a carrier of a sex-linked genetic disease (for example, hemophilia A).
  • Other family history of genetic disease. Couples who do not have an affected child but who have family medical histories of genetic or chromosomal abnormalities or are known through genetic screening to be carriers of an inherited disease are at increased risk of having an affected child. Prenatal testing is offered only when the suspected condition can be diagnosed before birth.

Precautions

Chorionic villus sampling usually is not recommended for:

  • A woman whose pregnancy has progressed further than about 12 weeks, counted from the first day of the last menstrual period.
  • A woman who is experiencing bleeding or spotting.
  • In the presence of certain vaginal infections.
  • When there are uterine abnormalities, such as a bicornuate (double) uterus or uterine fibroids.
  • Women who would not consider intervening in a preg- nancy where a genetic abnormality is detected.

Women who are concerned about the risk of miscarriage associated with CVS can be offered amniocentesis or a noninvasive alternative. One such alternative is a maternal blood test called triple marker screening or multiple marker screening, which is performed no sooner than 15 weeks but no later than 20 weeks of pregnancy. A sample of the pregnant woman's blood is analyzed for three substances produced by the fetus and passed into the mother's blood: alpha fetoprotein (AFP), human chorionic gonadotropin (HCG), and unconjugated estriol (UE3). Elevated AFP may indicate the presence of anencephaly or spina bifida; HCG is increased and AFP and UE3 are decreased in Down syndrome. This screening test, however, does not definitively diagnose a genetic defect and has a relatively high rate of false negatives, but it can help to identify an unborn baby at increased risk for these birth defects. A pregnant woman whose triple marker results are indicative of a fetus at risk would be encouraged to undergo further screening, such as ultrasound and/or amniocentesis (the timing of the triple marker screening rules out CVS) to provide a definitive diagnosis.

Since only cells and not fluid are collected, CVS does not allow evaluation of AFP to indicate the presence of neural tube defects. AFP is then generally measured by a maternal blood test at 15–20 weeks, but with less accuracy than would be obtained with an amniotic fluid sample.

A CVS sample may be obtained either via the vagina and cervix (transcervical CVS) or through the abdomen (transabdominal CVS). Woman who have a retroverted (tipped) uterus should be sampled transabdominally, since studies have shown the risk of miscarriage to be lower than when the procedure is done transcervically. In cases where the location of the placenta contraindicates transabdominal CVS, amniocentesis should be offered as an alternative screening test. Amniocentesis, however, is only rarely performed in the first trimester.

Although CVS is over 99% accurate in ruling out certain chromosomal birth defects and specific genetic problems, it is slightly more likely than amniocentesis to give inconclusive results. Amniocentesis, then, may be used as a back-up screening procedure. However, the patient should be cautioned that not all birth defects can be ruled out before birth and no prenatal test can guarantee the birth of a healthy baby. Because it is performed earlier in pregnancy than amniocentesis, CVS sampling is more likely to detect fatal genetic diseases that result in miscarriages after the diagnosis has been established.

Description

Chorionic villus sampling has been in use increasingly since the 1980s. The CVS procedure involves taking a sample of the chorion frondosum—the part of the chorionic membrane that contains the villi, microscopic, finger-like projections that emerge from the chorionic membrane to form the placenta—for laboratory analysis. The chorion is the outermost membrane surrounding the developing fetus; the amnion is the inner membrane that contains the amniotic fluid. The cells that make up the chorionic villi are of fetal origin, and, thus, normally have the same genetic makeup as the fetus. The sample may be obtained with a catheter, a thin, plastic tube, inserted through the vagina and the cervix (transcervical CVS) or with a needle through the abdominal wall (transabdominal CVS); both sampling methods take about five minutes to perform (not including preparation time), are equally effective, and carry similar risks.

Chorionic villus sampling is best performed between 10 and 12 weeks of pregnancy. Abdominal ultrasound is used to determine the position of the uterus, the position of the placenta within the uterus, and the size of the amniotic sack. The woman assumes the lithotomy position (on her back with her feet in stirrups). In transcervical CVS, the vulva, vagina, and cervix are thoroughly cleansed with antiseptic; no anesthetic is required. A speculum is inserted into the vagina and opened, then, using ultrasound as a guide, the doctor inserts a catheter through the cervix and into the uterus. The catheter is carefully advanced to the chorionic villi and suction is applied with the syringe attached to the catheter to obtain a small sample of the villi. The catheter is then carefully withdrawn.

In transabdominal CVS, the appropriate area on the woman's abdomen is cleansed thoroughly with antiseptic and a local anesthetic may be injected to numb the area. With ultrasound guidance to strictly avoid the placenta, a long needle is inserted through the woman's abdominal wall, through the uterine wall and to the chorionic villi. Suction is applied with the syringe attached to the needle to obtain a small sample of the villi, and the needle is then carefully withdrawn.

Most women report that transcervical CVS feels similar to the procedure for a Pap smear. The passage of the catheter through the cervix may cause cramping, and some women experience cramping or pinching sensations when the sample is taken. There is generally little or no discom- fort associated with the transabdominal procedure. Occasionally, when insufficient villus material is obtained, a second sampling procedure must be performed.

The chorionic villus sample is immediately placed into a sample dish containing nutrient medium for transport to the cytogenetics laboratory. At the laboratory, the sample is examined under the microscope so that any contaminating cells or material may be carefully removed. The villi can be analyzed immediately, or incubated for a day or more to give the cells time to undergo division. When the cells are in the midst of dividing, they are spread onto a slide and examined under a micro- scope. Cells that have clearly separated chromosomes are photographed to allow analysis of the type and number of chromosomes. The chromosomal images are collected in a report called a karyotype, which shows the number, shape, size, and arrangement of chromosomal pairs. For biochemical studies, deoxyribonucleic acid (DNA) is extracted from cultured chorionic villus cells. Depending upon which tests are performed, results may be available as early as two days or up to ten days after the procedure.

The chorionic villus sampling procedure costs about $3,000, including ultrasound, laboratory, and counseling charges. Some insurance plans may provide some level of coverage for this test.

Preparation

Thorough pre-CVS counseling is strongly recommended to give the couple the opportunity to make informed decisions about prenatal diagnosis. The couple should be provided with literature about CVS and genetic screening options to read prior to the counseling conference, so that any questions or concerns they may have can be addressed at that time. Prior to the procedure, the woman will likely be asked to sign a consent form.

For the procedure itself, the woman will be instructed to drink fluids and refrain from urinating so that the bladder is partly filled; excessive and uncomfortable bladder filling is not necessary. A filled bladder can help to properly position the uterus and create a better ultra- sound picture for guiding the CVS procedure.

Aftercare

After the sample is taken, the fetal heartbeat is checked by ultrasound before the woman leaves the examination room. A follow-up ultrasound procedure may be scheduled two to four days after CVS to ascertain the health of the fetus.

Most physicians recommend that the woman have someone drive her home after the CVS procedure and that she limit strenuous activity for the remainder of the day. A women who experiences excessive bleeding, vaginal discharge, fever , or abdominal pain after the procedure should consult her doctor.

Women with Rh negative blood may receive a Rho(D) immune globulin (RhoGAM) injection to avoid Rh incompatibility.

Advances in prenatal treatment have made it possible to treat some genetic defects diagnosed by prenatal screening before birth. Congenital adrenal hyperplasia (CAH), for example, is an inherited birth defect in which an enzyme deficiency causes a female fetus to develop abnormal external genitalia by the 16th week of pregnancy. Prenatal diagnosis by CVS in pregnancies known to be at risk of CAH allows the fetus to be treated with hormones during the critical period of development from 10 to 16 weeks of gestation and can circumvent the need for surgery after birth. For conditions in which prenatal treatment is not available, prenatal diagnosis can allow parents the opportunity to discuss their options with genetic counselors or other health care providers, to plan the delivery, and to prepare emotionally for the birth.

Complications

Of women who undergo transcervical CVS, one third experience minimal vaginal spotting and 7–10% experience vaginal bleeding; bleeding that is heavier than during a normal menstrual period should be reported to the doctor. One out of five women experience cramping following the procedure. Rupture of the amniotic membranes is a rare but serious complication that can lead to infection and/or miscarriage. The risk of miscarriage after CVS is 1–3%, compared to 0.5–1% for amniocentesis. A woman with Rh negative blood who is carrying an Rh positive fetus may be at an increased risk for developing Rh incompatibility following CVS and should be treated with Rho immune globulin.

In the early 1990s, there were several reports linking babies born with missing or shortened fingers or toes and abnormalities of the tongue and lower jaw with CVS performed before the tenth week of pregnancy. These reports raised concerns about the safety of CVS, although this type of limb defect is known to occur in approximately 1 out of every 1,700 babies. Subsequent studies of the risk of limb defects following CVS have produced conflicting results, and CVS safety studies continue. A study by the World Health Organization's CVS Registry, which performs ongoing assessment of CVS, reported in 1999 that the risk of these limb defects in babies born to more than 200,000 women who had CVS was not significantly increased compared to the norm. CVS is now generally performed only at ten weeks or later, and all women being offered CVS should be advised of the limited risk of limb defects.

There is risk with CVS of getting a "placental mosaicism" artifact in which the cultured cells contain some abnormal chromosomes that originate in the placenta and are not related to the fetus. Mosaicism occurs when cells have two or more distinct chromosome counts. This occurs when nondisjunction (failure of chromatids to separate) occurs in germ line cells (after fertilization). The fetus may be normal, and the only way to rule out actual mosaicism is to follow up with amniocentesis. There is also a risk that insufficient chorionic villi are collected for analysis, or that the cells collected are contaminated with cells of maternal origin. In this case, a second sampling procedure is performed about a week later, or amniocentesis may be offered as an alternative.

Results

The chorionic villus cells are cultured and photographed through a microscope during cell division to obtain images of the chromosomes. The images are sorted, identified, and reported in a karyotype. Humans have 23 pairs of chromosomes, including the sex chromosomes. The karyotype allows detection of aneuploidies (extra copies of chromosomes), chromosomal deletions, and gross chromosomal translocations. The gender of the fetus is identified from the sex chromosomes, and can be reported to the parents upon request.

DNA studies are performed when indicated by a family history of genetic disorders. DNA is the biochemical molecule that stores genetic information in the chromosomes. The DNA is extracted from the chorionic villus cells, and analysis of the DNA allows prenatal diagnosis of over 200 diseases, including Tay-Sachs disease, cystic fibrosis , sickle-cell anemia, and muscular dystrophy .

Health care team roles

The genetic counselor, physician, or other health care provider will provide information to the couple considering CVS and explain the procedure and its risks, and alternative procedures. Counselors can also advise parents of their options when they learn that their unborn child has an inherited disorder and/or help them prepare emotionally for the delivery of a child with a birth defect.

The obstetrics team carries out the ultrasound and CVS procedure, and ensures that the chorionic villus cells are properly handled for transport to a cytogenetics laboratory. The team of physician(s) and nurses also provide practical and psychological support for the couple undergoing prenatal diagnosis.

Technologists in the cytogenetics laboratory perform cell culture on the chorionic villus cells, fix a sample of cells during cell division, prepare the cells on a slide for microscopic analysis, and make photographs of the chromosomes of dividing cells to be collated in a karyotype. Cytogenetic technologists are also involved in the extraction of DNA from the cells and the biochemical testing of the DNA.

Training

The obstetrician receives special training in the procedure for collecting the chorionic villus specimen. Cytogenetic laboratory technologists, CLSp(CG) are specially trained in sterile techniques for cell culture,


KEY TERMS


Alpha fetoprotein —A serum protein produced by the fetus during pregnancy and passed to the moth- er's blood, useful in the prenatal diagnosis of multiple births or birth defects.

Amniocentesis —A prenatal screening procedure in which a sample of amniotic fluid from the amniotic sac in the uterus of a pregnant woman is obtained by inserting a needle through the abdominal wall.

Aneuploidy —An irregular number of chromosomes caused by the loss or addition of one or more chromosomes or parts of chromosomes.

Chromosomes —Tightly compressed rod-like structures that carry DNA; humans have 23 pairs of chromosomes including the sex chromosomes.

Cystic fibrosis —An inherited chronic disease of the exocrine glands, characterized by the production of viscous mucus that obstructs the pancreatic ducts and bronchi, leading to infection and fibrosis.

Down syndrome —Also called trisomy 21, a genetic disorder, associated with the presence of an extra copy or a rearrangement of chromosome 21, characterized by mild to severe mental retardation, weak muscle tone, a low nasal bridge, and epicanthic folds at the eyelids; formerly called mongolism.

Human chorionic gonadotropin —A hormone produced by the developing placenta that stimulates the production of estrogen and progesterone; its presence in maternal blood or urine is used to diagnose pregnancy.

Karyotype —A report in which photographic images of the chromosomes of a cell are displayed as a systematized arrangement of pairs in descending order of size. photomicrography, construction of the karyotype report, and biochemical analysis of DNA.

Muscular dystrophy —An inherited disease in which muscles become gradually wasted and are replaced by scar tissue and fat, sometimes also affecting the heart.

Neural tube defect —Any of a group of inherited abnormalities of brain and spinal cord development, including spina bifida and anencephaly, caused by failure of the neural tube to close prop- erly during development.

Rh incompatablility —An Rh negative person who is exposed to Rh positive red blood cells. When an Rh negative mother becomes pregnant with an Rh positive fetus, the fetal red blood cells may enter the maternal circulation and stimulate production of Rh antibodies. This process can be prevented by administration of Rho immune globulin prior to or during the pregnancy.

Sickle-cell anemia —An inherited chronic blood disease that occurs primarily among persons of African descent characterized by abnormal hemoglobin that causes red blood cells to become sickle-shaped and nonfunctional, leading to an enlarged spleen, chronic anemia, weakness, joint pain, and formation of blood clots.

Tay-Sachs disease —A rare and fatal inherited dis- ease occurring chiefly in persons of eastern European Jewish origin, characterized by a red spot on the retina, gradual blindness, and paralysis.

Ultrasound —Also called ultrasonography, a diagnostic imaging technique that uses reflected high- frequency sound waves to visualize internal body structures or organs, especially useful as a non- invasive prenatal diagnostic tool.


Resources

BOOKS

Carlson, Karen J., Stephanie A. Eisenstat, and Terra Ziporyn. The Harvard Guide to Women's Health. Cambridge, MA and London: Harvard University Press, 1996.

PERIODICALS

"Evaluation of chorionic villus sampling safety: WHO/PAHO Consultation on CVS." Prenatal Diagnosis 19, No. 2 (February 1999): 97–99.

OTHER

Arnot Ogden Medical Center website. Information About Chorionic Villi Sampling. 1995–1998. Arnot Ogden Medical Center. <http://www.aomc.org/cvs.html>.

Holbrook, R. Harold Jr., M.D. Chorionic Villus Sampling (CVS). <http://www.stanford.edu/~holbrook/>.

March of Dimes website. Chorionic Villus Sampling (CVS). 2001. March of Dimes Birth Defects Foundation, <http://www.modimes.org/HealthLibrary2/FactSheets/ChorionicVillusSampling.htm>.

Olney, Richard S., Cynthia A. Moore, Muin J. Khoury, J. David Erickson, Larry D. Edmonds, Lorenzo D. Botto. "Chorionic Villus Sampling and Amniocentesis: Recommendations for Prenatal Counseling." CDC WONDER Web publication MMWR 44(RR-9) (07/21/1995): 1-12, <http://www.cdc.gov>.

University of Maryland website. Chorionic Villus Sampling. 1998. A.D.A.M. Software, Inc., <http://umm.drkoop.com/conditions/>.

Patricia L. Bounds, Ph.D.

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"Chorionic Villus Sampling." Gale Encyclopedia of Nursing and Allied Health. . Retrieved September 21, 2018 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/chorionic-villus-sampling-1

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