What is Chorionic villus sampling?
Chorionic villus sampling (CVS) is a test available to pregnant women, particularly those with a family history of inherited disorders or who are over the age of 35.
It is an alternative to amniocentesis, to test for serious fetal problems, and CVS has the advantage that it can be done earlier than amniocentesis, at about ten weeks after fertilization. It takes around 20 minutes to perform.
At an early stage the embryo forms itself into two parts, one becoming the developing baby and the other developing into the placenta (afterbirth). The part that forms the placenta starts out as finger-like sections called chorionic villi, which burrow into the wall of the womb to come close to the mother’s blood vessels.
These villi are formed by the division of the original fertilized egg and so they have exactly the same DNA (genetic codes) as the embryo does, including any possible genetic abnormality. Any defect in one will be present in the other.
CVS is not entirely without risk. In around 2% of cases the procedure results in miscarriage (spontaneous abortion).
However, this can be weighed against the high natural risk of miscarriage during the first trimester: as many as 1 in 4 pregnancies.
Why chorionic sampling should be done
Chorionic villus sampling allows a very early diagnosis of serious genetic disorders. If the condition found cannot be treated, or if it causes severe disability in the child, the parents may decide to terminate the pregnancy.
This test also provides early notice for parents who choose to continue with the pregnancy, enabling them to start preparing for some of the challenges ahead.
Many fetal disorders can be detected by chorionic villus sampling. These include:
- Down’s syndrome and other chromosomal conditions,
- Cystic fibrosis, Duchenne’s muscular dystrophy and other disorders of the musculoskeletal system,
- Thalassaemia, sickle-cell anemia and other blood disorders,
- Antitrypsin deficiency, phenylketonuria and other metabolic disorders, and
- Fragile X syndrome, and other conditions causing mental health problems.
- Several other less common conditions can also be diagnosed early in this way.
When chorionic sampling should be done
The test is done at, or after, ten weeks of pregnancy. Very rarely it can be done earlier, but the risk of miscarriage is higher. This compares with amniocentesis, an alternative test for abnormalities. This is done at around 16 weeks and some results take up to four weeks to obtain.
This would mean that the pregnancy was at an advanced stage, around 20 weeks, before the options can be considered.
Chorionic villus sampling is recommended in pregnancies where there is a high risk of the baby having a serious inherited condition, for example when the woman has had previous pregnancies with fetal problems, if she is over the age of 35, or if there is a family history of conditions such as cystic fibrosis or Down’s syndrome.
How it is performed
In this test, a small sample of chorionic villi (placental tissues) is obtained either by passing a thin needle through the wall of the abdomen, or by passing a small tube through the vagina and the neck of the womb (cervix).
The instrument is moved to the site of the placenta under the guidance of ultrasound scanning. It is important to be able to ‘see’ the tip of the instrument using ultrasound, to avoid making a hole in the amniotic sac (the protective sac that cushions the baby with fluid as it grows).
From the cells obtained in this way, a chromosome (genetic) analysis can be done, and the results are usually available within 10 to 14 days.
Recovery after chorionic sampling
The test may be followed by a small amount of bleeding or cramps, but there are usually no other after-effects following a normal CVS procedure.
If high fever, excess bleeding or excess vaginal discharge occur, then medical advice should be sought.
It is possible for infection to occur, and miscarriage is a further possibility, for example if the amniotic sac has been punctured during the procedure.
In clinical trials, an association has been made between CVS and a slightly increased risk for limb abnormalities in the fetus (such as missing or short fingers and toes).
Should the test have a positive result, indicating that a condition has been detected, the parents will need counseling and support. Healthcare professionals should provide information on the condition and possible outcomes of the pregnancy, and there should be opportunity for full discussion of the options for termination or treatment (usually postnatal but certain conditions can be treated in the womb).