Imaging choice in the pregnant patient with abdominal pain

Diagnosis and principle. This is suspected acute appendicitis in a pregnant patient. The governing principle is ALARA applied to the mother-fetus unit, with the imaging order ultrasound first, MRI second, CT last, and no life-saving image withheld on radiation-dose grounds alone. The clinical question — is there a surgical intra-abdominal pathology — must be answered because a perforated appendix threatens both mother and fetus.^[3]

First-line: graded compression ultrasound of the right iliac fossa.^[3] Ultrasound is non-ionising, bedside, dynamic and repeatable, and is the correct first test in pregnancy. Limitations are operator dependence and reduced sensitivity in the third trimester and in high body mass index; a non-diagnostic scan does not end the workup. A pelvic ultrasound simultaneously evaluates the adnexa (torsion, ectopic) and the biliary tree.

Second-line: non-contrast MRI of the abdomen and pelvis. If ultrasound is non-diagnostic and the clinical concern persists, non-contrast MRI is the next step — superior soft-tissue contrast, no ionising radiation. Gadolinium is reserved for the indispensable study because it crosses the placenta and is excreted by the fetal kidney; if required, a group II macrocyclic agent (gadoterate, gadobutrol) is used, which carries negligible NSF risk at an eGFR of 30 mL/min/1.73 m² or above.^[2]

CT only if the first two are non-diagnostic and the decision hangs on it. A single diagnostic CT abdomen delivers a fetal dose typically under 50 mGy — doses at which no increase in lethality, teratogenicity, growth impairment or sterility has been demonstrated; the stochastic cancer risk is small relative to a missed appendicitis with perforation.^[5][6] CT dose-reduction protocols are applied; routine gonadal shielding is no longer best practice.^[5]

Contrast considerations. Iodinated contrast crosses the placenta in small amounts and is generally regarded as safe in pregnancy; it is given when diagnostic yield demands it. Gadolinium is avoided unless indispensable.^[3] The eGFR is checked before any contrast given the physiological changes of pregnancy.

Documentation and counselling. The rationale for the modality chosen (and rejected), the radiation and contrast considerations, and the discussion with the patient and the radiologist are documented. The surgical and obstetric teams are involved early.^[2]