Chorionic villus sampling, or CVS, is a prenatal test that samples part of the placenta to determine whether baby has a chromosomal condition, such as Down syndrome, or other genetic conditions, such as cystic fibrosis.
If you opt for the CVS procedure, it will be performed around the end of your first trimester, between weeks 10 and 12. The first step in the procedure is an ultrasound, which is used to confirm your pregnancy stage and determine the placenta’s location. Based on this, your doctor will determine whether to go through your cervix or abdomen to get the chorionic villi sample from your placenta. (This just depends on which direction will give easier access to your placenta.) The cells in this tissue have the same genetic makeup as baby, which means any chromosomal abnormalities that show up in the cells will also be present in baby.
For a transcervical procedure (through the cervix), you’ll be asked to lie on your back with your feet in stirrups. A speculum will be inserted into your vagina (think back to your last pap smear), and antiseptic will be used to clean your vagina and cervix. This ensures that bacteria won’t travel into your uterus during the procedure and cause an infection. Then, your doctor will thread a catheter (a long, thin tube) through your vagina and cervix and use gentle suction to take a tissue sample from the placenta.
For a transabdominal procedure (through the abdomen), you’ll receive a shot of local anesthetic to numb the belly area. Then, a longer needle will be put through your skin, muscle and uterine wall to reach the placenta and take a tissue sample. For each technique, ultrasound will be used to guide and monitor the entire process.
After the sample is extracted, baby’s heart rate and your blood pressure, pulse and breathing will be monitored for at least half an hour. If your blood is Rh negative, you'll receive a shot of immunoglobulin after (unless baby's father is also negative), in case your blood mixed with baby's potentially non-compatible blood during the test. Make sure someone is available to drive you home afterwards, because you'll need to take it easy for the rest of the day, and avoid sex, heavy lifting and flying for the next three days. You might experience minor cramping, but if it becomes severe or you notice leaking amniotic fluid or spotting, or develop a fever (a possible sign of an infection), call your doctor right away–these are all signs of potential miscarriage.
The test lasts about half an hour, and the actual tissue extraction only takes a few minutes. The transcervical procedure feels similar to a pap smear—there might be some cramping or pinching, but it shouldn’t last long. In a transabdominal procedure, you’ll feel a sharp but quick sting from the numbing medication, and no further pain when the collecting needle goes in. You might cramp a little when the needle is actually in your uterus.
Expect results in about seven to ten days. Cells from your tissue need to be isolated and allowed to reproduce for a week or so, then analyzed for any chromosomal abnormalities. CVS can also be used to detect certain genetic disorders, but not neural tube defects (you’ll need amniocentesis for that). Your chances of a mosaicism result, where some cells have abnormal chromosomes and some don’t, are about 1 percent. If this is the case, your doctor will probably recommend further testing, such as amniocentesis.
American College of Obstetrics and Gynecologists. Your Pregnancy and Birth. 4th ed. Washington, DC: ACOG; 2005.