Chorionic villus sampling (CVS) is an invasive diagnostic test which involves sampling the developing placenta late in the first trimester of pregnancy, to allow examination of the fetal karyotype and/or genotype. It is most commonly performed transabdominally, although it may also be performed transcervically prior to 13 weeks.
CVS is usually reserved for those women at increased risk of carrying a fetus with a chromosomal or genetic abnormality.
Annually about 5% of the pregnant population (approximately 30,000 women in the UK) are offered an invasive, prenatal diagnostic test, usually either amniocentesis or CVS.
Genetic counselling should ideally be offered prior to any pregnancy, when there is a family history of a condition which might be diagnosed either by amniocentesis or CVS. It is clearly important to avoid unnecessary invasive testing in pregnancy where possible.
Diagnostic testing should be provided within the context of informed consent and autonomy, both about the conditions being tested and about the implications for the continuation of the pregnancy. Pre- and post-test genetic counselling are both indicated.
It is important to remember that women choose whether or not to undergo CVS. The aim of the procedure is to provide chorionic material for prenatal diagnosis of chromosomal or single-gene abnormalities.
CVS is most likely to be used in the following situations:
- Positive antenatal screening test - eg, the combined test for trisomy.
- Past history of a genetic or chromosomal abnormality.
- Familial chromosomal rearrangement.
- Biochemical or molecular diagnosis of a familial genetic disorder.
CVS cannot be used to screen for structural problems such as neural tube defects, which have no known metabolic or molecular basis (unlike amniocentesis). Suspected neural tube defects should be investigated by biochemical and ultrasound markers.
See also separate Amniocentesis article.
- Active vaginal bleeding.
Transcervical route of sampling is also contra-indicated by:
- Cervical polyps.
- Fundal placenta.
- Retroverted uterus with posterior placement of placenta.
- CVS is usually performed between 11 and 13 weeks (11+0 and 13+6).
- Informed written consent should be obtained.
- Rhesus immunoprophylaxis should be given where appropriate (fetomaternal transfusion is a risk in amniocentesis and CVS).
- The placental sample is obtained either by ultrasound-guided transabdominal needle, or ultrasound-guided transcervical cannula aspiration or biopsy forceps
- Results are usually obtained within 7-14 days, although newer tests can reduce reporting time to 24-48 hours.
- Use of biopsy forceps rather than cannula aspiration for the transcervical route may be more effective and less painful for the woman but there is no difference in the risk of miscarriage.
- CVS allows diagnosis earlier in the pregnancy than with amniocentesis (which may only be safely conducted in the second trimester) and an earlier opportunity to consider termination of pregnancy in the event of fetal abnormality.
Diagnostic accuracy of CVS is 97.5-99.6%, depending on the abnormality being screened for. It is slightly less accurate than amniocentesis because of placental mosaicism (the placenta can have populations of cells with different karyotype/genotype in about 0.8-1.6% of cases). If results from direct preparation are inconclusive then amniocytes need to be cultured.
Risks and complications of chorionic villus sampling
- Sampling failure may occur due to laboratory failure, mosaicism, ambiguous results, insufficient sample or maternal cell contamination. A sample failure rate of 3% is regarded as the 'gold standard'.
- Miscarriage risk[6, 7]:
- Miscarriage following CVS is higher than after second-trimester amniocentesis; the background risk is higher earlier in pregnancy.
- There is evidence to suggest the risk is reducing, probably due to improvements in technique.
- In a low-risk population, background pregnancy loss is about 2%. Overall risk of miscarriage following CVS is about 3% but CVS is often performed in the presence of conditions associated with a higher rate of pregnancy loss.
- A systematic review and meta-analysis, which included only studies published since 2000 therefore reflecting current practice, suggests the procedure-related risks are much lower at 0.2%.
- Transcervical CVS is more technically demanding than the transabdominal route and more likely to lead to sample failures and multiple insertions, and to cause vaginal bleeding.
- Very experienced surgeons (more than 100 procedures per year) may have higher success rates and lower procedure-related miscarriages.
- On the whole, second-trimester amniocentesis is safer than transcervical CVS and less likely to result in laboratory failure.
- Where early diagnosis is needed, transabdominal CVS is the procedure of choice.
- Where transabdominal CVS is not technically possible then transcervical CVS or second-trimester amniocentesis should be used.
- Amniotic fluid leakage (0.3-0.7%).
- Vaginal bleeding occurs in 7% (higher for transcervical than transabdominal route).
- Sepsis (rare).
- CVS at 8-9 weeks has, in case reports, been linked with an increased incidence of fetal limb deficiencies (oromandibular limb hypoplasia and isolated limb disruption). The association between these abnormalities and early CVS is uncertain, as subsequent analyses haven't been able to confirm a link, although they generally included procedures performed after 10 weeks. CVS is no longer performed prior to 10 weeks. This is in part due to the greater technical difficulties involved.
Any benefits of earlier diagnosis with CVS must be carefully balanced against the slightly greater risk of pregnancy loss compared with second-trimester amniocentesis. There appears to be no significant difference in long-term health outcomes between children who had transcervical CVS or amniocentesis for prenatal testing.
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- Amniocentesis and Chorionic Villus Sampling; Royal College of Obstetricians and Gynaecologists (June 2010)
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- Akolekar R, Beta J, Picciarelli G, et al; Reply: To PMID 25042845. Ultrasound Obstet Gynecol. 2015 Jun 45(6):755-7. doi: 10.1002/uog.14851.
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- Schaap AH, van der Pol HG, Boer K, et al; Long-term follow-up of infants after transcervical chorionic villus sampling and after amniocentesis to compare congenital abnormalities and health status. Prenat Diagn. 2002 Jul 22(7):598-604.
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