Chorionic villus sampling (CVS)
• What is chorionic villus sampling?
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.
• 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.
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.
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.
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.
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.