Chorionic Villi Sampling (CVS) with Traditional Chromosome Analysis
To the Point: CVS is a diagnostic test that can tell you, with high accuracy, if your baby has an age-related chromosome abnormality such as Down syndrome by looking at the chromosomes in the chorionic villi cells, which are part of the placenta. In general, the chromosomes of placental cells are the same as that of the baby. CVS is done by inserting a catheter vaginally, or a very thin needle into the abdomen, into the chorionic villi of the placental tissue and removing a small amount of it. The needle and catheter are then immediately removed and the tissue is sent to the lab for analysis. CVS is generally considered fairly safe but not entirely risk-free; there is a risk of less than 1 in 200 (0.5%) for miscarriage associated with the procedure. The procedure is typically done at 10-13 weeks gestation and results take about 7-10 days.
Chorionic villus sampling (CVS) is a diagnostic test, meaning the results are typically “yes or no”. Chorionic villi are a part of the placenta, the structure that allows nutrients and waste to pass between mom and baby. The genetic information found in the placenta is typically the same as the genetic information found in the baby. The procedure itself is performed between 10-13 weeks gestation. There are two different methods of doing CVS: transabdominally or transcervically. Transabdominally involves inserting a needle into the abdomen to get some of the placenta, while transcervically means they have to insert a catheter into the vagina through the cervix.
The placental cells each have a set of chromosomes (see genetics 101 for more info). With CVS, we can essentially open up the cells and look directly at the chromosomes. In general, the chromosomes from the cells in the placenta are the same as the chromosomes in the baby. However, less than 1% of the time there may be parts of the placenta that are different. Often times this does not mean the baby has abnormal chromosomes, but rather that it is something that is isolated to the placenta (this is called confined placental mosaicism).
CVS can be done in the doctor’s office. Most women report that it can be uncomfortable, but most generally do not experience considerable pain. There is a risk of miscarriage, which for a long time was thought to be approximately 1/200. New data suggests that it is lower than that. Ultimately, over 99% of the time everything goes just fine. If you do have a CVS, you need to take it easy for 24-48 hours – ask your doctor for details. You will get results on all of the chromosomes, including the sex chromosomes, which will tell you if it is a boy or a girl .
CVS does not look for or rule out all birth defects or genetic conditions, but it does rule out chromosome abnormalities such as Down syndrome with over 99% accuracy. Historically, this test was offered only to high-risk pregnancies (e.g. advanced maternal age, abnormal screening test, etc.), however, new guidelines suggest that CVS be offered to all women, regardless of age.
CVS is a pretty quick procedure done in a doctor’s office. This procedure should only be performed by an experienced high-risk pregnancy doctor (like a perinatologist working a Maternal-Fetal Medicine clinic). The doctor uses ultrasound to find where the placenta is at and will determine whether the CVS will be transabdominal or transcervical. Continuing to use the ultrasound to guide, the doctor inserts a very thin needle either into the abdomen or the cervix until it enters the area where the placenta is. Because the uterus is a muscle, and the needle needs to pass into it to get to the placenta, some women can experience cramping during this part of the procedure. A small amount of tissue is removed by the catheter and a syringe, and then the needle is removed. The entire need part of the procedure generally takes a couple minutes or less.
The tissue is then sent off to the laboratory where the cells from the tissue are grown, and then the lab can look at the chromosomes. The chromosomes are evaluated for any extra (trisomy, like Down syndrome) or missing (monosomy, like Turner syndrome) chromosomes. Large deletions (missing sections of a chromosome) or duplications (extra sections of chromosome), and many different types of rearrangements can also be detected. The report the lab sends out has the results of all the chromosomes, including the sex chromosomes. So, if you don’t want to know if you’re having a boy or a girl, make sure to let your doctor know so they don’t spill the beans!
Unlike the amniocentesis, CVS does NOT look for evidence of open neural tube defects (ONTDs), such as spina bifida. There are other ways to look at the chances of ONTDs besides amniocentesis, including level II ultrasound in the second trimester, and a blood draw done on mom in the second trimester to measure something called maternal serum alpha-feto protein (MSAFP). Open neural tube defects occur in approximately 1/500 to 1/1000 babies born and chances of this type of condition do not increase as mom ages. For more information on ONTDs click here.
CVS results with traditional chromosome analysis are typically considered to be very accurate when looking for chromosome conditions, such as Down syndrome. Results are also very accurate if you are having the procedure to look for other specific genetic conditions based on family history, such as cystic fibrosis or Tay-Sachs disease. However, no test is perfect and there are rare cases when the results are hard to interpret and the doctor may recommend you consider amniocentesis. There are also rare cases of lab error.
With CVS chromosome results, there is a ~1% chance that the lab will see two different types of cells (for example, one set of cells that have the usual 46 chromosomes, and one set of cells that have an extra 21 chromosome, which causes Down syndrome). In this instance, we don’t know which set of cells the baby has, or if it has both too! In order to sort this out, amniocentesis is recommended.
With traditional chromosome analysis you will receive information on the chromosomes and sex, generally within about 10-14 days. Normal CVS results also do not rule out all genetic conditions or birth defects.
CVS was historically offered to women age 35 or older to look for Down syndrome, as this was one of the first conditions known to increase as mom’s age increases. As time has passed and technology has improved, CVS with traditional chromosome analysis is now capable of detecting a variety of chromosome conditions, ranging from trisomy 13 and trisomy 18 to Turner syndrome. No matter what condition has been diagnosed, we know that this is not necessarily an easy time or what you expected.
If your results indicate a chromosome condition or birth defect, it is often very helpful to take time to talk to your provider, a genetic counselor, friends and family, and even get a second opinion. Taking time and consulting with others is especially important if your results are confusing, unclear, or if you don’t know much about the condition that has been diagnosed.
Even if you know a bit about the condition, it’s often helpful to get up-to-date information as things have changed dramatically. For example, in the case of individuals with Down syndrome, quality of life and life expectancy is quite different than it was two generations ago. Also, depending on what website you go to, who you talk to, and what you read, you are going to find differing opinions on what life may be like for a child with the condition that has been diagnosed, what it is like to parent a child with the condition, etc. Some people benefit from finding someone who has a child with the same diagnosis and speaking with them directly. Your provider or genetic counselor may be able to help facilitate this.
Deciding whether or not to have a diagnostic test, such as CVS, is a big decision. For some it may be a fairly easy call. For others it may be agonizing. Either way, it deserves thoughtful consideration.
Patients often ask their doctors or genetic counselors, “would you have the CVS?” This is a great question. However, the answer is different for each individual and circumstance. One of the most important aspects of prenatal genetic counseling is that whatever the patient decides is based on their personality, beliefs and values, not the providers. Instead we offer you some questions to consider: