This procedure samples part of the developing placenta during the first trimester of pregnancy (usually between 11-13 weeks), allowing examination of the fetal karyotype and/or genotype. It can be performed by transabdominal or transcervical routes, the former being able to be performed beyond 13 weeks also.

Indications

The aim of the procedure is prenatal diagnosis of chromosomal/single-gene abnormalities.

Contra-indications

• Active bleeding
• Infection
• Transcervical route of sampling: cervical polyps, fibroids, fundal placenta, retroverted uterus with posterior placement of placenta

Previous or suspected neural tube defects should be investigated by biochemical and ultrasound markers.

Practical details

• Procedure is usually performed between 11 and 13 weeks (11+0 and 13+6).²
• Written consent should be obtained.
• Sample obtained either by ultrasound-guided transabdominal needle, or transcervical cannula aspiration (CA)/biopsy forceps (BF).
• Results are obtained within 7-14 days (direct preparation 1 week, long-term culture 2 weeks).
• Thus, CVS allows earlier diagnosis than with amniocentesis (which may only be safely conducted in the second trimester) and earlier opportunity to consider termination of pregnancy in the event of a fetal abnormality.
• Use of BF rather than CA for transcervical route seems to make the procedure technically easier, and is preferred by patients and operators alike.³

Diagnostic accuracy is 97.5-99.6%, depending on 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.

Adverse effects

• Repeat procedures are required in 1-10% of cases due either to laboratory failure, mosaicism, ambiguous results, insufficient sample or maternal cell contamination. The latter occurs more frequently in transcervical CVS (1.9%-3.8%).
• In a low-risk population, background pregnancy loss is about 2%, and second trimester amniocentesis increases this risk by a further 1%.
• Miscarriage rates following CVS are higher than after second trimester amniocentesis. However, there is some evidence to suggest this continues to improve over time, probably relating to improvements in technique.⁴
• The increase in risk is significantly higher for transcervical CVS in terms of pregnancy loss and miscarriage.⁵
• Transcervical CVS is more technically demanding than transabdominal route and more likely to lead to sample failures and multiple insertions.⁵
• On the whole, second trimester amniocentesis is safer than transcervical sampling or early amniocentesis.⁵
• Where early diagnosis is needed, transabdominal CVS is preferred to early amniocentesis/transcervical CVS.⁵
• Where transabdominal CVS is not technically possible then transcervical CVS or second trimester amniocentesis should be used.⁵
• Amniotic fluid leakage 0.3-0.7%.
• Vaginal bleeding 7% (higher for transcervical vs. transabdominal route).
• Sepsis (rare).

Earlier chorionic villus sampling procedures, i.e. before 10 weeks

CVS at 8-9 weeks was reported to be associated with an increased incidence in limb deficiencies (oromandibular limb hypoplasia and isolated limb disruption) in the fetus. Therefore procedures are no longer performed at this early gestation. The association between limb abnormalities and early chorionic villus sampling is uncertain; however, some believe CVS has distinct teratogenic effects.⁶ There are reports of earlier CVS being carried out in exceptional circumstances, but there was higher fetal loss and a case of limb abnormality.⁷

Any benefits of earlier diagnosis with CVS must be carefully balanced against the 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.⁸

Document references

1. Stranc LC, Evans JA, Hamerton JL; Chorionic villus sampling and amniocentesis for prenatal diagnosis. Lancet. 1997 Mar 8;349(9053):711-4.
2. Amniocentesis and Chorionic Villus Sampling, Royal College of Obstretricians and Gynaecologists (June 2010)
3. von Dadelszen P, Sermer M, Hillier J, et al; A randomised controlled trial of biopsy forceps and cannula aspiration for transcervical chorionic villus sampling. BJOG. 2005 May;112(5):559-66. [abstract]
4. Caughey AB, Hopkins LM, Norton ME; Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstet Gynecol. 2006 Sep;108(3 Pt 1):612-6. [abstract]
5. Alfirevic Z, Sundberg K, Brigham S; Amniocentesis and chorionic villus sampling for prenatal diagnosis. Cochrane Database Syst Rev. 2003;(3):CD003252. [abstract]
6. Brambati B, Tului L; Chorionic villus sampling and amniocentesis. Curr Opin Obstet Gynecol. 2005 Apr;17(2):197-201. [abstract]
7. Wapner RJ, Evans MI, Davis G, et al; Procedural risks versus theology: chorionic villus sampling for Orthodox Jews at less than 8 weeks' gestation. Am J Obstet Gynecol. 2002 Jun;186(6):1133-6. [abstract]
8. 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. [abstract]

Internet and further reading

• Amniocentesis and Chorionic Villus Sampling, Royal College of Obstretricians and Gynaecologists (June 2010)

Acknowledgements