Chorionic villus sampling — also called trophoblast sampling or biopsy, and internationally CVS (Chorionic Villus Sampling) — is an invasive method for the prenatal diagnosis of certain fetal disorders.
The test is performed between the 11th and the 14th week of pregnancy and consists of taking chorionic villi from the placenta, at the point where it is attached to the wall of the uterus. The chorionic villi are small projections of the placenta and contain cells with the same genetic material (DNA) as the fetus, since they come from the same original fertilized egg.
Diagnostic value and indications
It diagnoses more than 200 chromosomal abnormalities, congenital metabolic disorders and single-gene (genetic) diseases — for example thalassemia and sickle cell anemia, Tay-Sachs disease, cystic fibrosis and Down syndrome. The diagnostic accuracy of the method is estimated at 98%. Because the chromosome analysis also determines the sex of the fetus, it can help identify sex-linked disorders (certain forms of muscular dystrophy, for instance, which occur only in boys).
Its advantage is the earlier diagnosis of fetal abnormalities, which allows earlier and therefore safer intervention and gives more time for the appropriate decisions to be made.
That intervention may be either termination of the pregnancy, if the abnormalities are severe or incompatible with life, or intrauterine treatment of the fetus where this is possible.
THE PROCEDURE
For the transabdominal approach the mother lies flat on her back. We first use ultrasound to check the position of the fetus and the placenta. The skin of the lower abdomen is cleaned with antiseptic. We then give a local anesthetic to the area so that the mother feels no pain. A relatively fine needle (17G–18G) is then introduced under ultrasound guidance through the skin as far as the point where the placenta is attached.
Through this needle, the syringe attached to it aspirates the fragments of tissue that the needle detaches by the appropriate manoeuvres (a combination of movement and negative pressure). The whole examination takes 15 minutes; the sampling itself takes about 5 minutes and is done on an outpatient basis, so under normal circumstances no hospital stay is needed.
RESULTS — the material obtained is sent to a genetics laboratory, where the cells are cultured in a special medium before being collected for analysis. The result of the test is available after about 8–15 days. Early results are obtained within 2–3 days from a rapid analysis (Amnio QF-PCR), which rules out the commonest chromosomal abnormalities (Down, Edwards, Patau and Turner syndromes) and identifies the sex of the fetus.
When testing for single-gene disorders, the time needed for the results depends on how common the disorder is (for thalassemia, for example, about 7 days are needed).
The test can rarely give false positive results in cases of mosaicism — that is, an abnormal picture for a fetus that is essentially healthy. Rarely the test can fail, because suitable material was not obtained or because the sample was contaminated with the mother's cells.
Complications and contraindications — symptoms that may indicate a complication are heavier vaginal bleeding or loss of fluid, fever and lower abdominal pain (sometimes felt as contractions of the uterus).
The risk of miscarriage from chorionic villus sampling is about 1%, the same as the risk of amniocentesis performed at 16 weeks. That risk is considered even lower when the procedure is carried out by a properly trained and certified doctor.
The contraindications to the test are chiefly: pre-existing complications of the pregnancy (particularly vaginal bleeding), an active infection and, as a relative contraindication, uterine fibroids.
In Rhesus-negative women a dose (300 µg) of Anti-D (Anti-Rh) immunoglobulin is given intramuscularly after the procedure as a precaution, to prevent Rh sensitization.
INSTRUCTIONS AFTER CHORIONIC VILLUS SAMPLING — after the procedure you should avoid the following:
- Travelling for several hours immediately after the procedure.
- Work for 48 hours. You do not need to stay lying down during this time, but it is best to stay at home and avoid any strenuous activity. Moving around the house and light tasks (preparing food, for example) are allowed.
- Sexual intercourse for about 20 days.
In the first day or two after the procedure you may see a spot of blood on your underwear. This should not worry you. You should, however, contact your doctor in the following situations:
- If you see large amounts of fresh blood
- If you have severe pain in the lower abdomen
- If you develop a fever
For the 4 days after the test you should take the following medication:
- Amoxil 500 mg capsules (one capsule every 8 hours)
- If you are Rhesus negative you will need an intramuscular injection of Rhophylac within 48 hours, which you will also obtain from the pharmacy.