Imaging in Pregnancy: A Referrer's Guide

The most important message about imaging in pregnancy is easily lost: a clinically necessary study should not be withheld because the patient is pregnant. Missed or delayed diagnoses - pulmonary embolism and appendicitis are the classic examples - harm both mother and fetus far more reliably than the radiation dose of a single diagnostic study.

The referrer's task is therefore not avoidance but optimisation: choose the modality that answers the question with the least fetal exposure, document pregnancy status so the imaging team can tailor the protocol, and frame the risk honestly for an often-anxious patient.

Australian practice follows the same principles as international guidance: ultrasound and MRI are preferred where diagnostically adequate; X-ray and CT are used when they are the right test, with dose-optimised technique. This guide covers the general principles - consult current RANZCR guidance and obstetric advice for individual decisions.

Ultrasound is the first-line modality for most imaging questions in pregnancy. It involves no ionising radiation, has an extensive safety record across decades of obstetric use, and answers many of the common questions directly - abdominal pain, pelvic pathology, renal colic assessment, deep vein thrombosis, and of course fetal assessment itself.

MRI is the usual second line when ultrasound is inconclusive and a definitive answer matters - suspected appendicitis with a non-diagnostic ultrasound being the archetypal example. MRI uses no ionising radiation, and no harmful fetal effects have been demonstrated at standard clinical field strengths. Historic caution about first-trimester MRI has softened; current consensus is that MRI can be performed at any stage of pregnancy when clinically indicated, though many services still prefer to defer plainly non-urgent studies.

The practical caveat is that gadolinium contrast is generally avoided in pregnancy - discussed below - so pregnant-patient MRI is usually performed without contrast.

Patients - and sometimes clinicians - dramatically overestimate the fetal risk of diagnostic X-ray and CT. The doses at which deterministic fetal harm (malformation, growth restriction, intellectual impairment) has been observed are far above the fetal dose from almost any single diagnostic study. A chest X-ray or a CT of the head or chest delivers a negligible dose to a fetus that is well outside the primary beam; even studies that directly irradiate the pelvis remain well below recognised harm thresholds.

The residual consideration is a small theoretical increase in lifetime childhood cancer risk, which informs modality choice but does not prohibit a necessary scan.

When a pregnant patient needs a CT - suspected pulmonary embolism being the common scenario - the honest framing is: "The scan we're recommending involves a very small radiation dose, far below levels known to cause harm, and missing this diagnosis would be much more dangerous for both of you." Clinics will apply dose-reduction protocols and shielding where appropriate.

Iodinated contrast (CT) crosses the placenta, but studies have not shown teratogenic effects, and the main theoretical concern - transient effects on the neonatal thyroid - has not translated into demonstrated clinical harm. Where a contrast CT is genuinely indicated in pregnancy (such as CT pulmonary angiography), contrast is used; neonatal thyroid function screening, already routine in Australia, provides reassurance.

Gadolinium (MRI) is treated more cautiously. It crosses the placenta and persists in amniotic fluid, and observational data have raised unresolved questions, so gadolinium is generally avoided in pregnancy unless the benefit clearly outweighs the uncertainty - a radiologist-level decision.

For lactation, the guidance is reassuring for both agent types: only a tiny fraction of an intravenous dose reaches breast milk, and even less is absorbed by the infant. Current guidance holds that breastfeeding can continue normally after either iodinated contrast or gadolinium; mothers who remain concerned can express and discard for 24 hours, but this is optional rather than required.

Pregnancy-related documentation on the referral protects the patient and streamlines the appointment:

• Pregnancy status - confirmed pregnancy with gestation, or "possibly pregnant", or a documented negative status • Last menstrual period (LMP) date for any patient of childbearing potential referred for X-ray, CT or nuclear medicine - many clinics apply specific rules around booking timing based on it • For confirmed pregnancy: gestation, and any obstetric input already obtained • Breastfeeding status where contrast may be used • The clinical urgency - a suspected PE workup reads very differently from a routine musculoskeletal request

If pregnancy status is uncertain, say so rather than guessing; clinics routinely perform point-of-care testing before studies involving pelvic irradiation. Omitting pregnancy information doesn't avoid the issue - it simply moves the conversation to the clinic reception desk, with the patient unprepared for it.

A quick orientation to frequent presentations - always individualised, and ideally discussed with the radiology practice or obstetric team when in doubt:

• Suspected pulmonary embolism - imaging should not be delayed; local protocols choose between CT pulmonary angiography and ventilation-perfusion scanning, often guided by chest X-ray findings and local expertise • Suspected appendicitis - ultrasound first; MRI where available if ultrasound is inconclusive; surgery should not be delayed for imaging in a clinically clear case • Renal colic - ultrasound first; low-dose CT is reserved for unresolved diagnostic uncertainty • Suspected DVT - Doppler ultrasound, as outside pregnancy • Trauma - imaging as clinically required; necessary trauma imaging is never withheld for pregnancy

The consistent thread: pregnancy changes the order of preference, not the obligation to reach a diagnosis. For patient-facing information you can share, see our guide on medical imaging during pregnancy.