Chorionic Villus Sampling
Chorionic villus sampling (CVS) is a test conducted during the first trimester in order to screen for chromosomal abnormalities that could result in birth defects such as Down syndrome. It can also detect Tay-Sachs disease or cystic fibrosis, among other conditions. The procedure involves taking a tissue sample from the chorionic villi, which are tiny, finger-like projections that line the chorion, one of the membranes that separate the mother from the fetus. These villi contain the baby’s DNA, and analyzing them can tell your doctor a great deal about the baby’s genetic makeup. In simpler terms, CVS means obtaining a very tiny piece of the placenta for DNA analysis.
Why Chorionic Villus Sampling is Done
Chorionic villus sampling is used when there is reason to believe that a fetus may be at risk for certain genetic abnormalities. For example, a positive result from NIPT (non-invasive prenatal testing—see below). It can also be used for paternity testing, if there is doubt about the identity of the baby’s father, although this indication is rarely used.
Other possible reasons for chorionic villus sampling include:
- Abnormality on an early sonogram, such as during a Nuchal Translucency scan.
- Previous pregnancy affected by Down syndrome or other chromosomal condition
- Mother over the age of 35
- Family history of genetic or chromosomal abnormalities
Many prospective parents opt to have chorionic villus sampling done because they want to know beforehand whether their child will have a chance to live a normal, healthy life. Faced with the possibility that their child might have a condition such as Down syndrome, some mothers choose not to continue with the pregnancy. Even if you are certain you would never make that decision, it is still important to be well informed about your baby’s health as early in the pregnancy as possible, so that you can be prepared for the challenges that may lie ahead.
How Chorionic Villus Sampling Works
There are two ways of doing chorionic villus sampling: transcervical and transabdominal. Transabdominal chorionic villus sampling—the more common method of testing—involves inserting a needle through the abdomen in much the same way that amniocentesis is done. With transcervical chorionic villus sampling, on the other hand, the sample is taken through the vagina.
What to Expect
Chorionic villus sampling is typically done between the 10th and 13th weeks of pregnancy. The procedure usually takes about 30 minutes and is not painful, though it is generally uncomfortable. You will be asked to arrive with a full bladder, if possible (this makes it easier to see the fetus and placenta during ultrasound). You may also be asked to sign a consent form indicating that you understand the possible risks involved with chorionic villus sampling (see below).
To begin, your doctor will perform an ultrasound test in order to verify the baby’s gestational age and to determine the location of the placenta. The ultrasound will be used as a guide when taking the tissue sample, meaning that most practitioners perform CVS while watching the ultrasound monitor in real-time. What happens next depends on which type of chorionic villus sampling procedure is being performed:
For a transabdominal CVS procedure, you will be given a local anesthetic to numb your belly, and a needle will be inserted into your abdomen. The doctor will use the ultrasound images to guide the placement of the needle, which is attached to a syringe, through which a small sample of chorionic villus tissue will be taken.
Your doctor may recommend against transabdominal CVS if the placenta is positioned in such a way that the baby blocks access to it, but this is rare.
For a transcervical CVS procedure, a thin tube is inserted into the vagina and through the cervical opening guided toward the placenta by the ultrasound image. Your doctor may recommend that transcervical CVS not be performed if you have recently been experiencing vaginal bleeding, if the placenta is partially blocking the cervical canal, if you have a cervical infection, or in the case of certain sexually transmitted diseases.
After the procedure you may experience some light bleeding and/or cramping. The spotting can last from hours to days. Severe cramps or heavy bleeding is not normal, and your obstetrical care provider should be notified right away if this occurs.
Risks of Chorionic Villus Sampling
As with almost any medical procedure, there are risks associated with chorionic villus sampling. The most serious of these risks is the possibility of miscarriage, which occurs approximately 1 percent of the time (the risk of miscarriage is slightly higher with transcervical CVS). And of course, as with any medical procedure, there is always the risk, however slight, of infection.
It is also possible that an amniotic fluid leak may develop following chorionic villus sampling. Luckily most fluid leaks stop on their own, and the fluid re-accumulates. If membrane rupture occurs at this early stage of the pregnancy, however, the risk of infection is high enough that termination of the pregnancy might be advised.
If your blood is Rh-negative and the baby’s father has Rh-positive blood, there is a possibility that you may become Rh sensitized during the course of the test, which puts you and your baby at risk for a condition known as Rh disease, which causes your immune system to attack your baby. The standard management for these situations is to administer Rhogam (anti Rh D antibody) to the mother to prevent this from taking place.
Is CVS Reliable?
Chorionic villus sampling is considered to be a very reliable test, but there is approximately a 1% chance of false positive results. One reason is that certain genetic conditions may exist in the placenta but not in the fetus itself, such as mosaicism of the DNA. This might require an amniocentesis at week 16–17 for confirmation.
Alternatives to Chorionic Villus Sampling—NIPT
NIPT—non-invasive prenatal testing—is frequently employed as an alternative to CVS. This is a blood test done on the pregnant woman that is able to analyze fetal DNA present in the mother’s bloodstream. The best aspect of this test is that it is “non-invasive,” meaning that there is no risk to the fetus.
When considering CVS for screening, many experts suggest having NIPT done instead. However, any positive NIPT result will likely prompt your doctor to recommend chorionic villus sampling (or amniocentesis, if it is later in the pregnancy).