What is the risk of CT (Computed Tomography) radiation to the fetus during pregnancy?

CT Radiation Risk to the Fetus During Pregnancy

CT radiation poses minimal risk to the fetus when exposure is below 50 mGy, which is the case for most diagnostic CT scans, but risk increases with higher doses and should be carefully considered in the risk-benefit analysis.

Radiation Dose Thresholds and Associated Risks

• Fetal radiation exposure below 50 mGy is not associated with detectable increases in adverse fetal outcomes 1
• Exposure between 50-500 mGy shows increased risk of adverse fetal effects 1
• Doses >500 mGy may be catastrophic to both mother and fetus 1

Specific Risks by Dose Level:

• 50 mGy: Associated with doubled relative risk of childhood cancer 2, 1

• 100 mGy: Increased risk of mental retardation (estimated at 0.025 IQ point loss per mGy above 100 mGy) 2, 1

• 150 mGy: Increased risk of malformations 2

• 200 mGy: Suggested as possible reason for considering pregnancy termination 2

Typical Fetal Radiation Exposure from Common CT Procedures

Procedure	Typical Fetal Dose
CT abdomen/pelvis	13-25 mGy [2]
CT chest	0.3 mGy [2]
Head or chest CT	Minimal (fetus not in direct beam) [3]

Important Context:

• CT scans where the fetus is not directly in the beam (head, chest) result in minimal exposure and need not factor heavily in risk-benefit analysis 3
• Abdominal/pelvic CT delivers the highest fetal radiation dose among diagnostic procedures 4
• For perspective, the natural background risk of childhood cancer is 1-2.5 per 1000, and exposure to 50 mGy increases this by only 1.1-3 per 1000 2

Alternative Imaging Modalities

• First-line options: Ultrasound and MRI (without gadolinium) are preferred during pregnancy 1
• Ultrasound: Recommended as first-line for suspected gynecologic or obstetric conditions 1
• MRI without gadolinium: Provides excellent soft tissue contrast without ionizing radiation 1
  • Avoid gadolinium as it crosses the placental barrier 2, 1

When CT Is Necessary During Pregnancy

If CT is clinically indicated (e.g., suspected pulmonary embolism, trauma assessment):

1. Apply the ALARA principle (As Low As Reasonably Achievable) 2, 1
2. Use radiation dose reduction techniques 1
3. Consider timing: If possible, avoid non-urgent radiologic testing during weeks 8-15 of gestation (period of highest CNS sensitivity) 1
4. Use abdominal shielding when appropriate 1

Special Considerations

• Venography: Simulation studies calculated radiation to the fetus during unilateral venography with abdominal shielding to be 0.05 rads and 0.32 rads when shielding is removed 2
• Contrast agents: Iodinated contrast should be used only when absolutely necessary; gadolinium-based contrast agents should be avoided 1
• Documentation: Recording radiation dose in medical records is highly recommended, particularly if the fetus is in the field of view 2

Conclusion

When clinically indicated, CT scans should not be withheld during pregnancy, as the risk from most diagnostic CT procedures is minimal compared to the potential benefit of accurate diagnosis. However, alternative non-ionizing imaging modalities should be considered first when appropriate, and radiation dose should be minimized following established protocols.