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Chorionic villus sampling is an early and accurate prenatal test that diagnoses Down syndrome and many other genetic disorders. Some women choose CVS over an amnio because it can be done as early as 10 weeks.
What is chorionic villus sampling?
Chorionic villus sampling (CVS) is a prenatal test that diagnoses chromosomal abnormalities such as Down syndrome, as well as a host of other genetic disorders. The doctor takes cells from tiny fingerlike projections on your placenta called the chorionic villi and sends them to a lab for genetic analysis.
CVS is an alternative to amniocentesis. They both produce a karyotype – a picture of your baby's chromosomes – so that your caregiver can see for sure if there are any problems. (You can also find out your baby's gender if you want to.)
Women who choose to have CVS or amniocentesis are often those at increased risk for genetic and chromosomal problems, in part because these tests are invasive and carry a small risk of miscarriage.
The main advantage of CVS over amniocentesis is that you can have it done earlier — generally between 10 and 13 weeks of pregnancy. For an amnio, you'll have to wait until you're at least 15 weeks pregnant.
What kind of problems does CVS diagnose?
Like amniocentesis, CVS can identify:
- Nearly all chromosomal abnormalities, including Down syndrome, trisomy 13, trisomy 18, and sex chromosome abnormalities (such as Turner syndrome). The test can diagnose these conditions, but it can't measure their severity.
- Several hundred genetic disorders, such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease. The test does not automatically look for all of them, but if your baby is at increased risk for one or more of these disorders, your doctor can request that the test results show whether he has the disease.
CVS does not detect neural tube defects, such as spina bifida. The most accurate way to diagnose neural tube defects is during a second-trimester ultrasound.
How accurate is CVS?
With CVS you have a 98 to 99 percent chance of getting accurate results and a 1 to 2 percent chance of unclear results. This is called a confined placental mosaicism, in which some of the cell lines cultured from the placenta contain abnormal chromosomes and some are normal. If your CVS detects a mosaicism, you'll have to have amniocentesis and possibly other testing to determine whether your baby is affected.
What factors would put me at higher risk of having a baby with a genetic abnormality or disorder?
Some factors to consider include:
Down syndrome screening results
You had a first trimester screening that indicated your baby is at a higher risk for Down syndrome or another chromosomal problem. First-trimester screenings include the nuchal translucency scan (NT scan) and cell-free fetal DNA testing (noninvasive prenatal testing, or NIPT).
An early ultrasound showed that your baby may have structural defects associated with a chromosomal problem.
Carrier screening results
You and your spouse are both carriers of a recessive genetic disorder such as cystic fibrosis or sickle cell disease.
You've previously been pregnant with a child with a genetic abnormality and may be at a higher risk of this happening again.
Your family history
You or your partner has a chromosomal abnormality or genetic disorder or a family history that puts your child at increased risk for genetic problems.
Anyone can have a baby with a chromosomal abnormality, but the risk rises with the mother's age. For example, your likelihood of carrying a baby with Down syndrome ranges from about 1 in 1,200 at age 25 to 1 in 100 at age 40.
What are the risks of CVS?
According to the American College of Obstetricians and Gynecologists, the risk of miscarriage with CVS at 10 weeks or later is 1 in 455. This is slightly higher than the risk of miscarriage after amniocentesis, which is 1 in 769. Some recent studies shown even lower risk rates.
Because a certain percentage of women will end up miscarrying at this point in pregnancy anyway, there's no way of knowing for sure whether a miscarriage following CVS was actually caused by the procedure. Your particular risk depends in large part on the skill and experience of the doctor performing the procedure.
Some older studies found that CVS may have caused defects in a baby's limbs, but this was mostly seen in tests done on women before 9 weeks of pregnancy. Current research suggests that there is no increased risk for this problem in women who have CVS at 10 weeks or later.
Is there any way to reduce the risks of CVS?
• Ask your practitioner or genetic counselor to refer you to an experienced doctor who does a lot of CVS procedures.
• Ask about the estimated rate of procedure-related miscarriage for the doctor or the center where you're considering having the procedure done.
• Make sure that an experienced registered diagnostic medical sonographer provides continuous ultrasound guidance during the procedure. This greatly increases the chances that the doctor will be able to obtain enough tissue on the first try, so you won't have to repeat the procedure.
Can I meet with a counselor before deciding whether to have CVS?
Yes. In fact, most testing centers require that you meet with a genetic counselor to discuss the risks and benefits of various methods of prenatal screening and testing before you have an invasive procedure like CVS or amniocentesis. The counselor will take down your family history and ask questions about your pregnancy.
Your answers will enable the counselor to give you a sense of your risk for having a baby with chromosomal problems or a particular genetic disease. Then you can decide whether you want to be screened, go right for CVS or amnio, or forgo testing altogether.
How can I decide what's right for me?
The American College of Obstetricians and Gynecologists recommends that women of all ages be offered genetic screening and diagnostic testing options. Your practitioner or genetic counselor should discuss the pros and cons of the available approaches with you. But ultimately, whether or not to test is a personal decision.
• No screening or testing. Some women decide to forgo all screening and testing because the results are not going to influence the management of their pregnancy. They would never terminate the pregnancy if a serious problem were found. Some women are not willing to take even a very small risk of miscarriage in order to find out the results of diagnostic testing.
• Screening. Many women opt for screening and then make a decision about diagnostic testing based on the initial results. If you opt for screening first, you can then decide — with the help of your practitioner or genetic counselor — whether your results indicate a high enough risk that you want to have CVS or amnio to determine whether a problem exists.
• Diagnostic testing. Some women opt for diagnostic testing after screening, and some choose diagnostic testing right away. Women who choose diagnostic testing right away may know that they're at high risk for a chromosomal problem or a condition that can't be detected by screening — or they may just feel that they want to know as much as possible about their baby's condition and are willing to live with the small risk of miscarriage to find out.
There are even a few conditions that may be treated while the baby is still in the womb. So, if there's a strong suspicion that your baby may have one of these rare problems, you might opt for CVS to find out.
Some women who opt for invasive testing are pretty clear in advance that they would terminate the pregnancy if a serious problem were found. Others feel that finding out that their baby will have special needs will help them prepare emotionally for the challenges ahead. In certain cases, they may want to switch to a better-equipped hospital with specialists.
There's no one right decision. Individual parents-to-be have different feelings about what risks are acceptable and may arrive at different conclusions when facing the same set of circumstances.
How would I decide between CVS and amniocentesis?
Both tests can tell you whether your baby has a chromosomal problem or certain genetic disorders. Considerations include:
• Timing. CVS is done earlier in pregnancy (as early as 10 weeks), so you can find out sooner about your baby's condition. If everything's okay, your mind will be put at ease that much sooner. Or, if there is a serious problem and you opt to terminate the pregnancy, you'll be able to do so while you're still in the first trimester. On the other hand, you may prefer to wait for the results from your second-trimester anatomy ultrasound before subjecting yourself to an invasive test. At that point, amniocentesis would be your only option.
• Specific health concerns. For example, if noninvasive prenatal testing (NIPT) suggests you're at high risk for having a baby with Down syndrome, you may want to wait to see if there's an abnormal second-trimester anatomy ultrasound. If that shows an abnormality, you might decide to have amniocentesis.
• Risks. CVS is generally thought to have a slightly higher miscarriage rate than amniocentesis, but this may not be the case everywhere. Medical centers that perform a lot of these procedures may have similarly low miscarriage rates for both. However, only a small percentage of doctors perform CVS, so in some areas it may be difficult to find an experienced specialist who does the procedure.
Before making a decision, you'll want to discuss all of these issues with your partner, your health practitioner, and possibly a genetic counselor.
What's it like to have CVS?
Before you have CVS, you'll have an ultrasound to confirm how far along you are and to make sure it's possible to get a good sample for the test. (Some testing centers do this when you come in for your CVS, while others do it in advance.) You'll need to have a full bladder in order to get a good view of your uterus on the ultrasound.
The goal of CVS is to obtain a tiny tissue sample from your placenta, which will be sent to a lab for analysis. The doctor withdraws the sample through either your cervix (transcervical CVS) or your abdomen (transabdominal CVS), depending on which approach gives her best access to your placenta. A technician uses ultrasound to help guide the procedure.
For transcervical CVS, your vagina and cervix are first cleaned with an antiseptic. This is done to prevent any bacteria from entering the uterus, which could cause an infection. The doctor then threads a catheter through the cervix and uses gentle suction to get a sample from the placenta.
For transabdominal CVS, you may get a shot of local anesthetic in that area first. Then the doctor inserts a needle through your skin, muscle, and uterine wall to extract the sample from the placenta.
Whether the tissue is obtained through your cervix or your abdomen, the amniotic sac where your baby is growing won't be disturbed. When the test is done, the doctor checks your baby's heartbeat with an external fetal monitor or by ultrasound.
The procedure might hurt a little, but it's over relatively quickly. It takes about 30 to 45 minutes from start to finish and the extraction itself takes only a few minutes. Some women who've had a transcervical CVS say it feels similar to a pap smear, which may feel like a cramp or pinch. Those who have a transabdominal CVS may experience some pain or discomfort in the abdominal area.
Note: If your blood is Rh-negative, you'll need a shot of Rh immunoglobulin after CVS (unless your baby's father is Rh-negative as well) because your baby's blood may have mixed with yours during the procedure and it may not be compatible.
What happens after the procedure?
You'll need to take it easy for the rest of the day, so arrange for someone to drive you home.
• Avoid intercourse and strenuous activities for the next two or three days.
• Some practitioners recommend avoiding swimming or baths as well if the CVS was done through your cervix.
• Don't plan any travel. Flying isn't risky, but it's a good idea to stay close to home for a few days in case you have any symptoms that need to be checked out.
• Call your doctor or midwife if you have unusual symptoms. Some cramping and light bleeding over the next day or so are both normal but report them to your doctor or midwife anyway. If you have significant cramping or vaginal spotting, or you're leaking amniotic fluid, call your practitioner immediately. They could be signs of impending miscarriage. Also call right away if you have a fever, which could be a sign of an infection.
When will I get the results?
Some of your preliminary test results may be available in a couple of days, and some of the testing will take two to four weeks.
Why so long? For the chromosomal analysis, technicians at the laboratory isolate the tissue cells (which have the same genetic makeup as the baby's cells) and allow them to reproduce for a week or two. Then they analyze the cells for chromosomal abnormalities.
What happens if my baby is found to have a problem?
You will be offered genetic counseling and a consultation with a maternal fetal medicine specialist so that you can get more information and discuss your options. Some women opt to terminate the pregnancy, while others decide to continue.
Whichever route you choose, you may find that you want further counseling or support. Some women find support groups helpful, others may want individual counseling, and some may choose both. Be sure to let your practitioner and genetic counselor know if you need more help so they can give you the appropriate referrals.