Complete Guide to Chorionic Villus Sampling (CVS)

Chorionic Villus Sampling (CVS) is an important prenatal diagnostic procedure that allows for the early detection of various genetic and chromosomal conditions in a developing baby. While not routinely offered to all expectant parents, understanding what CVS involves, when it might be recommended, and what information it can provide is valuable knowledge, particularly for those with identified risk factors. This comprehensive guide explains the CVS procedure, its benefits, potential risks, and what to expect throughout the process.

What is CVS

Chorionic Villus Sampling (CVS) is a prenatal diagnostic test that involves removing and testing a small sample of cells from the placenta (the chorionic villi). The placenta is the organ that links the mother's blood supply with the developing baby, and it contains cells with the same genetic makeup as the baby. By examining these cells, healthcare providers can detect various genetic or chromosomal conditions such as Down's syndrome, Edwards' syndrome, or Patau's syndrome.

Unlike some other prenatal tests, CVS can be performed relatively early in pregnancy, typically between the 11th and 14th weeks, although it can sometimes be done later if necessary. This provides expectant parents with information about their baby's health at an earlier stage compared to other diagnostic procedures.

CVS can be used to diagnose a wide range of genetic and chromosomal conditions, including:

• Down's syndrome – a condition where children have some degree of learning disability and delayed development, though this varies widely between individuals
• Edwards' syndrome and Patau's syndrome – conditions that can result in miscarriage, stillbirth, or severe physical problems and learning disabilities
• Cystic fibrosis – a condition where the lungs and digestive system become clogged with thick, sticky mucus
• Duchenne muscular dystrophy – a condition that causes progressive muscle weakness and disability
• Thalassaemia – a blood disorder that affects the production of hemoglobin, which can cause anemia, restricted growth, and organ damage
• Sickle cell disease – a condition where red blood cells develop abnormally and cannot carry oxygen properly throughout the body
• Phenylketonuria – a metabolic disorder where the body cannot break down the amino acid phenylalanine, which can build up to dangerous levels in the brain

It's worth noting that CVS cannot detect neural tube defects such as spina bifida. These are typically identified through ultrasound scans or other prenatal tests.

When it's Offered

CVS is not a routine test offered to all pregnant individuals. It is specifically recommended when there is a higher chance that the baby might have a genetic or chromosomal condition. Your healthcare provider might suggest CVS if:

• An antenatal screening test has indicated an increased risk of conditions such as Down's syndrome, Edwards' syndrome, or Patau's syndrome
• You have had a previous pregnancy affected by a genetic condition
• You or your partner has a family history of a genetic condition that could be inherited, such as sickle cell disease, thalassaemia, cystic fibrosis, or muscular dystrophy
• You are in an older age group, which increases the risk of chromosomal abnormalities

If you're considering CVS, your healthcare team will provide comprehensive counseling to help you understand the purpose of the test, the procedure itself, potential risks involved, the accuracy of the results, and what the possible outcomes might mean for you and your baby.

Generally, you won't need to make any special preparations before the test. You can eat and drink as normal beforehand. In some cases, you may be advised to have a full bladder for the ultrasound guidance, but your healthcare provider will inform you about this before your appointment if necessary.

It's often helpful to arrange for someone to accompany you to the appointment for emotional support, and potentially to drive you home afterward if you feel uncomfortable.

The Procedure

The CVS procedure typically takes about 10 minutes to complete, though the entire appointment may last around 30 minutes to accommodate pre-procedure counseling and post-procedure monitoring. There are two main methods for performing CVS:

Transabdominal CVS

This is the most common approach:

• Your abdomen will be cleaned with an antiseptic solution
• A local anesthetic injection may be used to numb the area
• Guided by continuous ultrasound scanning, a thin needle is inserted through your abdominal wall into the womb
• The needle is directed to the placenta, carefully avoiding the amniotic sac and the baby
• A small sample of cells from the chorionic villi is collected using a syringe attached to the needle
• The needle is then removed

Transcervical CVS

In some cases, the sample may be collected through the cervix:

• A speculum is inserted into the vagina to visualize the cervix
• Guided by ultrasound, a thin catheter or small forceps is inserted through the cervix
• The catheter is directed to the placenta to collect a small sample of cells
• The catheter or forceps is then removed

The method chosen typically depends on the position of the placenta and which approach allows for easier access. The transabdominal method is generally preferred as it's often easier to perform and may have a slightly lower risk of complications such as vaginal bleeding immediately after the procedure.

The procedure is usually described as uncomfortable rather than painful. For transabdominal CVS, you might feel a stinging sensation when the local anesthetic is injected, and some pressure or discomfort when the needle is inserted. Transcervical CVS is often compared to the feeling of having a cervical screening test (Pap smear).

After the Procedure

Following CVS, you'll likely be monitored for up to an hour to ensure there are no immediate complications. Here's what to expect after the procedure:

• You may experience mild cramping similar to period pain for a few hours
• Light vaginal bleeding or spotting can occur, particularly after transcervical CVS
• You can take over-the-counter pain relievers such as paracetamol (but not ibuprofen or aspirin) if you experience discomfort
• It's advisable to avoid strenuous activity for the rest of the day
• Most women can resume their normal activities the following day

Results

After the CVS procedure, the collected sample is sent to a laboratory for testing. The cells are analyzed to count the number of chromosomes and check their structure. If CVS is being performed to test for a specific genetic condition, the cells will also be tested for this.

The timeframe for receiving results includes:

• Initial results (rapid CVS result) are typically available within about 3 days
• A more detailed set of results will be available within 2 weeks

If the rapid CVS result and a previous ultrasound scan both indicate a condition, your doctor will discuss your options with you immediately. However, if your previous ultrasound did not find anything unexpected, it's generally recommended to wait for the more detailed results before making any significant decisions.

CVS is estimated to give a definitive result in 99 out of every 100 women having the test. In a very small number of cases, it may not be possible to get a conclusive result, either because the sample was too small or because there are differences between the cells in the placenta and those in the baby (a rare occurrence known as placental mosaicism). In such cases, additional testing such as amniocentesis might be recommended.

Potential Risks

While CVS provides valuable diagnostic information, it's important to understand the potential risks associated with this invasive procedure:

• Miscarriage: The most significant risk is miscarriage (pregnancy loss before 23 weeks). The chance of miscarriage after CVS is estimated to be less than 1 in 200 (0.5%) for singleton pregnancies, and around 1 in 100 (1%) for multiple pregnancies such as twins.
• Infection: There is a small risk of developing an infection, though this occurs in less than 1 in 1,000 procedures.
• Inadequate sample: In approximately 1 in 100 procedures, the sample collected may be insufficient for testing, potentially requiring a repeat procedure.
• Rhesus sensitization: If you have Rh-negative blood and your baby has Rh-positive blood, there's a small chance that the procedure could cause your blood to mix with your baby's blood, leading to Rh sensitization. To prevent this, you may be given an injection of anti-D immunoglobulin.

It's worth noting that it can be difficult to determine which miscarriages would have happened anyway and which are a direct result of the CVS procedure. Recent research suggests that only a very small proportion of miscarriages occurring after CVS are directly attributable to the procedure itself.

Alternatives

If you're concerned about the risks associated with CVS, you might consider these alternatives:

• Amniocentesis: This is another diagnostic test that samples amniotic fluid from around the baby. It's typically performed between the 15th and 18th weeks of pregnancy. The miscarriage risk is similar to CVS (about 1 in 200), but the pregnancy will be more advanced before you can get results, giving you less time to consider your options.
• Non-invasive Prenatal Testing (NIPT): This blood test analyzes cell-free fetal DNA in the maternal bloodstream and can screen for common chromosomal conditions with high accuracy. While not diagnostic, it has a higher detection rate and lower false positive rate than traditional screening tests, with no risk of miscarriage.
• Detailed Ultrasound: Advanced ultrasound scanning can sometimes identify physical markers associated with certain genetic conditions.

The most appropriate testing option depends on your specific situation, the timing of your pregnancy, and the conditions you're most concerned about. A genetic counselor or maternal-fetal medicine specialist can help you understand which approach best meets your needs.

Deciding whether to undergo CVS is a deeply personal choice. Consider factors such as how important it is for you to know definitively about your baby's health before birth, whether this information would change your pregnancy management, what level of procedural risk you're comfortable accepting, and what support systems you have in place. Take time to discuss these considerations with your partner, healthcare provider, and perhaps a genetic counselor before making your decision.