What are the differences between Chorionic Villus Sampling (CVS) and amniocentesis?

Key Differences Between Chorionic Villus Sampling (CVS) and Amniocentesis

Amniocentesis is generally safer than CVS, with lower rates of pregnancy loss and no associated risk of limb deficiencies, making it the preferred diagnostic procedure for most prenatal genetic testing scenarios. 1

Timing of Procedures

• CVS is typically performed earlier in pregnancy (10-12 weeks' gestation) compared to traditional amniocentesis (15-18 weeks' gestation), allowing for earlier diagnosis and decision-making 1
• Early amniocentesis (11-14 weeks) is sometimes performed but has not been fully assessed for safety compared to standard second-trimester amniocentesis 1, 2
• The earlier timing of CVS can be advantageous when early diagnosis is critical, as termination in the first trimester carries lower maternal morbidity and mortality than second-trimester procedures 1, 3

Sample Collection Methods

• Amniocentesis involves removing a small sample of amniotic fluid surrounding the fetus, which contains cells shed from the fetal skin, bladder, gastrointestinal tract, and amnion 1
• CVS involves biopsying placental cells (chorionic villi) that are derived from the same fertilized egg as the fetus, using either a transcervical or transabdominal approach 1
• Transabdominal CVS appears to be safer than transcervical CVS, with fewer technical failures (1.1% vs 2.0%) 1, 2

Safety Considerations

Miscarriage Risk

• Amniocentesis at 15-18 weeks increases the risk of miscarriage by approximately 0.25%-0.50% (1/400-1/200) 1
• CVS carries a higher miscarriage risk of approximately 0.5%-1.0% (1/200-1/100) 1
• Early amniocentesis (before 15 weeks) has been shown to have higher pregnancy loss rates (7.6% vs 5.9%) compared to second-trimester amniocentesis and is not recommended as a safe early alternative 2, 4

Birth Defect Risk

• CVS has been associated with transverse limb deficiencies at a rate of 0.03%-0.10% (1/3,000-1/1,000) 1
• The risk and severity of limb deficiencies from CVS are related to timing:
  • CVS performed before 10 weeks has a higher risk (0.20%) 1
  • CVS performed at or after 10 weeks has a lower risk (0.07%), with most defects limited to the digits 1
• No increased risk of limb deficiencies has been observed with amniocentesis performed at 15-18 weeks 1
• Early amniocentesis has been associated with increased risk of talipes (clubfoot) compared to CVS (1.8% vs 0.2%) 4, 3

Diagnostic Accuracy

• Both procedures provide cells for karyotyping, which is the standard and definitive means of diagnosing fetal aneuploidy 1
• CVS has been associated with more sampling failures, technical difficulties, and false positive/negative results compared to amniocentesis 5, 2
• Chromosomal mosaicism detected by CVS may represent a phenomenon inherent to placental tissue and may require follow-up amniocentesis for confirmation 6, 2

Clinical Considerations

• The background risk for major birth defects (with or without chromosomal abnormalities) for women of all ages is approximately 3% 1
• For women aged 35, the risk of having a child with Down syndrome is approximately 0.3% (1/385), increasing to 3% (1/30) by age 45 1
• When early diagnosis is required but transabdominal CVS may be technically difficult, options include transcervical CVS in the first trimester or second-trimester amniocentesis 4, 2
• Termination of pregnancy without confirmatory diagnostic testing following a positive non-invasive prenatal test (NIPT) is not recommended due to the risk to euploid pregnancies 3, 2

Pitfalls and Caveats

• Never perform CVS before 10 weeks' gestation due to significantly increased risk of limb deficiencies 1
• Early amniocentesis (before 15 weeks) should be avoided due to increased risks of pregnancy loss and talipes 2, 4
• Patients should be strongly counseled against termination of pregnancy based solely on NIPT results without confirmatory diagnostic testing 3, 2
• The benefits of earlier diagnosis with CVS must be weighed against the greater risk of pregnancy loss compared to amniocentesis 5, 2