ObstetricsTier 2Topic

Risks associated with various investigative procedures, particularly imaging, during pregnancy

Principle

Balances diagnostic necessity against fetal radiation/contrast exposure - the guiding principle is that an indicated maternal investigation should not be withheld out of unfounded radiation fear, since untreated maternal disease is often the greater fetal risk
Background risk of childhood cancer from any pregnancy is low; radiation risk to the fetus is dose- and gestation-dependent, and most diagnostic imaging falls well below thresholds of concern

Deterministic effects (malformation, growth restriction, microcephaly) - threshold ~100-200 mGy, essentially never reached by diagnostic imaging
Stochastic effects (childhood cancer) - no threshold, risk proportional to dose, most sensitive in the first trimester
Typical fetal doses: CXR <0.01 mGy, CT abdomen/pelvis ~10-20 mGy, CT pulmonary angiogram <1 mGy (lower fetal dose than a V/Q scan, though V/Q gives less maternal breast dose)

Ultrasound and MRI (without gadolinium) - no known fetal risk, first-line where feasible
Iodinated contrast - crosses the placenta, theoretical neonatal thyroid suppression risk - considered low-risk and used when clinically indicated; check neonatal TSH if given
Gadolinium - crosses the placenta, associated with rare adverse outcomes in some cohort data - avoided unless essential
V/Q scan vs CTPA for suspected PE in pregnancy - both acceptable; V/Q gives a lower fetal dose, CTPA gives more diagnostic certainty and a lower maternal breast dose

Do not delay or avoid a clinically indicated CT, X-ray or nuclear medicine study because of pregnancy - use the lowest-dose protocol and shield where practical, but proceed
Radiology/medical physics consultation for cumulative or high-dose exposures

Hyperthyroidism investigation is limited in pregnancy - radioactive iodine uptake scanning is contraindicated, use ultrasound instead

Anxiety about imaging is a more common clinical problem than actual harm - most diagnostic studies carry negligible measurable fetal risk

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