Chorionic villus sampling (CVS)
What is chorionic villus sampling?
Chorionic villus sampling (CVS) is a prenatal test that detects chromosomal abnormalities such as Down syndrome. This is done by analyzing the genetic makeup of cells taken from tiny fingerlike projections on the placenta called the chorionic villi. Its main advantage over amniocentesis is that you can have it done earlier — generally between 10 and 12 weeks of pregnancy, although some testing centers will do it at 13 weeks. (For an amnio you have to wait until you’re 15 to 18 weeks pregnant, in your second trimester.)
What kind of problems does the test detect?
CVS is approximately 99 percent accurate at detecting hundreds of genetic disorders and chromosomal abnormalities — but it’s not used to look for every one of those in every woman. It’s used to check for certain disorders, such as Down syndrome, in everyone who’s tested. But it’s used to look for certain other problems — such as Tay-Sachs disease, cystic fibrosis, or sickle cell anemia — only if your baby is thought to be at risk for them. The test can’t detect neural tube defects, such as spina bifida.
There’s a 1 percent chance of getting a false positive result called a “confined placental mosaicism,” in which some of the cells cultured from the placenta contain abnormal chromosomes but the fetus is normal. So if your CVS detects a mosaicism, you’ll have to have amniocentesis to determine whether your baby is affected by it.
Is this test appropriate for me?
While most centers will do CVS for any pregnant woman who wants it, the procedure isn’t offered routinely to all pregnant women because it carries a small risk of miscarriage. About one in one-hundred women will miscarry because of the test. It’s usually only offered to those who are found to have a somewhat increased risk of having a baby with a chromosomal abnormality. You may be in this category if:
- You’re going to be 35 years old or older on your due date. The risk of having a child with a chromosomal defect increases as a woman ages. The chance that your baby will have a trisomy disorder like Down syndrome is about one in about 250 when you’re 35 years old.
- You’ve previously been pregnant with a child with a chromosome problem or another birth defect.
- 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. Or you and your spouse are both carriers of a recessive genetic disorder such as cystic fibrosis or sickle cell disease.
You may be able to have a noninvasive screening test (the nuchal translucency or first trimester combined screening) to assess your baby’s risk of chromosomal abnormalities before you decide whether to have CVS.
What are the risks of CVS?
In deciding whether to have this test, you’ll need to weigh your desire to know about your baby’s condition with the slight risk that the test will lead to a miscarriage. You’ll definitely want to discuss your options with your doctor so that you understand all the risks and benefits of your choices.
You’ll want to weigh the small risk that the procedure will cause you to miscarry. According to the Centers for Disease Control and Prevention (CDC), the risk of miscarriage from CVS is between one in 100 and one in 200, depending on the skill and experience of the doctor performing it. This risk (0.5 to 1 percent) is only slightly higher than that of amniocentesis (0.25 to 0.5 percent), and that’s mostly because there’s always a higher risk of miscarrying in the first trimester than in the second.
There has also been some concern that CVS is associated with limb defects in babies, such as missing fingers or toes, but this was primarily in tests done on women before their tenth week of pregnancy. Most studies have found no increased risk for this problem in women who have CVS at 10 weeks or later. These are all things you should discuss with your partner, your doctor, and a genetic counselor.
Can I meet with a counselor before deciding whether to have the test?
The perinatal center will discuss the risks and benefits of various methods of prenatal testing before you have the test. 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 a problem and determine whether you need to be screened for a particular genetic disease. Then you can decide whether you want to go ahead with the CVS.
What’s the procedure like?
Before having your 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.
The goal of the procedure is to obtain a tiny tissue sample of your placenta, which will be sent to a lab for analysis. Depending on where your placenta is attached to your uterus, the doctor performs either a transcervical or a transabdominal CVS, withdrawing the sample of chorionic villi cells through either your cervix or your abdomen. She does this with a catheter (a thin tube) or a needle, assisted by a technician who uses a vaginal or abdominal ultrasound to determine the position of the placenta and better guide the needle or catheter during the procedure. (If done abdominally, the ultrasound exam might be a little uncomfortable because your bladder has to be full.)
For a transcervical CVS, which is more common, the doctor cleans your vagina and cervix with an antiseptic, to prevent any bacteria from being carried by the catheter into the uterus, where it could cause an infection. She then threads the catheter through the cervix, using gentle suction to get a sample from the placenta. For a transabdominal CVS, the doctor numbs a spot on your abdomen with a shot of local anesthetic. She then inserts a longer needle through your skin, muscle, and uterine wall to extract the sample.
Whether you have a transcervical or transabdominal CVS, 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 no longer than a half hour from start to finish and the extraction itself runs only a few minutes. Women who’ve had a transcervical CVS say it feels similar to a pap smear, which may not hurt at all or may feel like a cramp or pinch. Those who undergo a transabdominal CVS may experience some 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 it’s possible that your baby’s blood mixed with yours during the procedure.
What happens after the CVS?
Prenatal tests can be pretty draining, and CVS is no exception. You’ll need to take it easy immediately afterward, so arrange for someone to drive you home. Rest for the remainder of the day and abstain from strenuous physical activity, sex, and exercise for the next three days. You may have some cramping and light bleeding, which is normal, but report it to your doctor or midwife anyway. If you notice fluid leaking from your vagina or have a fever, call your caregiver immediately.
Then sit tight. The technicians at the laboratory will 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’ll test the cells for chromosomal abnormalities. (You can also find out your baby’s gender if you want to!) You should have the results in seven to ten days, although some labs can give you preliminary findings in two days. If that option is available, you’ll have to make a special request to receive this speedy but more costly service.
How do I decide whether to have a CVS or wait for an amnio?
The CVS may be a better choice if you’re at risk for having a baby with a chromosomal problem and you want to know your baby’s condition during your first trimester, either so you can terminate the pregnancy early if necessary or just to cut down on worrying time. If you wait for the amnio at 15 weeks, then 17 weeks is the soonest you’d have the results.
Amniocentesis may be a better choice if you decide you want to wait for the results of a multiple marker screening (usually done between 15 and 20 weeks) before subjecting yourself to a more invasive test. The CVS must be done by 12 or 13 weeks to be effective.