Does amniocentesis (amniotic fluid test) increase the risk of miscarriage?

Amniocentesis and Risk of Miscarriage

Yes, amniocentesis does increase the risk of miscarriage, with procedure-related miscarriage rates estimated at approximately 0.25%-0.50% (1 in 400 to 1 in 200) above the background risk of pregnancy loss. 1

Understanding the Risk

Amniocentesis is a prenatal diagnostic procedure that involves removing a small sample of amniotic fluid surrounding the fetus. This fluid contains cells shed from the fetal skin, bladder, gastrointestinal tract, and amnion, which can be analyzed for genetic testing.

The risk of miscarriage following amniocentesis has been established through controlled studies:

• Traditional estimates from older studies cited in CDC guidelines indicate an approximately 0.5% increased risk of miscarriage following amniocentesis 1
• One randomized trial indicated the amniocentesis-related miscarriage rate may be as high as 1% 1
• More recent systematic reviews suggest the procedure-related risk may be lower than previously thought, with estimates around 0.35% (95% CI: 0.07 to 0.63) 2
• A 2019 retrospective cohort study found an even lower procedure-related risk that was not statistically significant compared to women who did not undergo the procedure 3

Risk Factors and Timing Considerations

Several factors influence the risk profile of amniocentesis:

• Timing of procedure: Standard amniocentesis is typically performed at 15-18 weeks' gestation 1

• Early amniocentesis (performed before 15 weeks) carries higher risks than standard second-trimester amniocentesis, including:

  • Increased total pregnancy losses (7.6% vs 5.9%) 4
  • Higher incidence of congenital anomalies, particularly talipes (clubfoot) 4
  • Not considered a safe early alternative to standard amniocentesis 4

• Operator experience: Risk varies by center and practitioner experience 1

• Ultrasound guidance: Modern amniocentesis is performed under continuous ultrasound guidance, which has improved safety compared to older techniques

Comparison with Chorionic Villus Sampling (CVS)

When considering invasive prenatal testing options:

• CVS (typically performed at 10-12 weeks) has a procedure-related miscarriage rate of approximately 0.5%-1.0% (1/200-1/100) 1
• The difference in fetal loss rate between CVS and amniocentesis has been estimated to be about 0.8% higher after CVS, though this difference was not statistically significant in some studies 1
• More recent systematic reviews suggest the procedure-related risk for both procedures may be similar at around 0.35% 2

Important Considerations

• Background risk: Any assessment of procedure-related risk must account for the background risk of spontaneous pregnancy loss, which is approximately 2-3% in the second trimester 1, 3
• Indication for testing: The benefit of obtaining diagnostic information must be weighed against the procedure-related risk
• Infection risk: While infection (chorioamnionitis) is a potential complication, it occurs rarely (<0.1%) after amniocentesis 1

Clinical Implications

When counseling patients about amniocentesis:

• Provide accurate information about the small but real risk of procedure-related miscarriage
• Consider the patient's individual risk factors for chromosomal abnormalities
• Discuss alternative testing options, including non-invasive prenatal testing (NIPT)
• Ensure the procedure is performed by experienced operators using continuous ultrasound guidance to minimize risk

The decision to undergo amniocentesis should be made after careful consideration of the risks and benefits, with the understanding that while the procedure does increase the risk of miscarriage, this risk is relatively small with modern techniques.