Are weekly ultrasound scans safe and recommended during the first trimester?

Safety of Weekly Ultrasound Scans in First Trimester

Weekly ultrasound scans are generally safe during the first trimester, but they should be performed only when medically indicated rather than routinely.

Indications for Weekly First Trimester Scans

Weekly ultrasound monitoring in the first trimester is appropriate in specific high-risk scenarios:

1. Monochorionic twin pregnancies with complications:

   • After laser therapy for Twin-Twin Transfusion Syndrome (TTTS), weekly surveillance is recommended for 6 weeks 1
   • For Stage I Twin Anemia-Polycythemia Sequence (TAPS) presenting before 32-34 weeks, close monitoring is generally preferred 1

2. High-risk singleton pregnancies:

   • History of recurrent pregnancy loss
   • Threatened miscarriage with bleeding
   • Uncertain pregnancy viability

Safety Considerations

Ultrasound in the first trimester is considered safe when performed for medical indications:

• No evidence suggests that diagnostic ultrasound causes harm to the developing embryo or fetus when used appropriately
• The American College of Radiology (ACR) does not specify safety concerns with repeated first-trimester ultrasounds 1
• Modern ultrasound equipment uses the ALARA principle (As Low As Reasonably Achievable) to minimize exposure

Recommended First Trimester Ultrasound Schedule

For normal, low-risk pregnancies:

• First scan: Dating and viability assessment (7-10 weeks)
• Second scan: Nuchal translucency (NT) measurement at 11-13+6 weeks 1

For high-risk pregnancies:

• Monochorionic twins: More frequent monitoring may be needed, especially with NT discordance ≥20% or CRL discrepancy ≥10%, which are associated with 30% risk of severe TTTS 1
• Post-treatment monitoring: Weekly surveillance for 6 weeks after interventions like laser therapy 1

Important Considerations

• Five of six guidelines recommend against routine third-trimester scans in low-risk women 1, and by extension, routine weekly first-trimester scans
• The Society for Maternal-Fetal Medicine recommends that ultrasound surveillance for TTTS begin at 16 weeks (not in first trimester) for monochorionic diamniotic twin pregnancies 1
• First trimester is important for determining chorionicity and amnionicity in multiple gestations 1

Potential Pitfalls

• Overdiagnosis: Frequent scanning may lead to detection of transient findings that resolve spontaneously
• Patient anxiety: Frequent scans without clear medical indication may increase maternal anxiety
• Resource utilization: Unnecessary scans strain healthcare resources
• Limited utility: Many complications of multiple gestations like TTTS typically develop after 16 weeks, not in the first trimester 1

Conclusion

While weekly ultrasound scans in the first trimester are generally safe from a physical perspective, they should be performed only when medically indicated rather than as a routine practice. For most low-risk pregnancies, two well-timed scans in the first trimester are sufficient, while high-risk pregnancies may require more frequent monitoring based on specific clinical scenarios.