Chorionic Villus Sampling

Synonym - CVS

Sampling a part of the developing placenta during the first trimester of pregnancy (usually between 10-13 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.

Aim of procedure - Prenatal diagnosis of chromosomal/single-gene abnormalities.

• Previous or suspected neural tube defects (as not the appropriate test)
• Active bleeding
• Infection
• Transcervical route of sampling: cervical polyps, fibroids, fundal placenta, retroverted uterus with posterior placement of placenta

• Procedure is usually performed at 10-13 weeks.²
• Written consent should be obtained.
• Sample obtained either by ultrasound-guided transabdominal needle, or transcervical cannula aspiration/biopsy forceps.
• 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 biopsy forceps rather than cannula aspiration for transcervical route seems to make the procedure technically easier, and to be preferred by patients and operators alike.³

Diagnostic accuracy 97.5-99.6%, depending on abnormality being screened for. It's 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.

• 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 is 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)

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 foetus and so procedure no longer performed this early. The association between limb abnormalities and early chorionic villus sampling is uncertain, but 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