Imaging in Pregnancy: Safety and Decision-Making

Pregnancy imaging decisions are built around two facts. First, ultrasound and MRI avoid ionising radiation and therefore become preferred whenever they can answer the clinical question. Second, most diagnostic radiographs and many justified CT studies still deliver fetal doses well below the level associated with deterministic effects such as malformation or neurodevelopmental injury.

That means the real task is not to avoid imaging at all cost. The real task is to match modality to question, optimise dose when ionising radiation is used, and counsel patients honestly. Orthopaedic practice most often encounters this in trauma, suspected pelvic or spinal injury, infection, and severe back pain with neurological compromise.

Diagnostic imaging is only half of the orthopaedic problem. If the pregnant patient has a fracture that needs fixing, the image intensifier will be on for the duration of the operation, and this is the part of the pathway that is most under a surgeon's own control.

The most useful finding here is counter-intuitive. In a series of eight pregnant women undergoing open reduction and internal fixation of an acetabular fracture between 5 and 26 weeks' gestation, intraoperative pelvic fluoroscopy averaged 39 seconds and conferred the LEAST fetal exposure of any imaging in the pathway — while the pre-operative pelvic CT conferred the most, exceeding 50 mGy in every case that required one. All eight pregnancies reached 36 weeks with normal Apgar scores and seven of eight reductions were anatomic. Evidence level 4: small retrospective case series with calculated rather than measured fetal doses.

Two things follow. First, the place to economise on fetal dose is the pre-operative CT protocol, not the fixation — hobbling the fluoroscopy to protect the fetus buys almost nothing and risks a malreduced acetabulum, which is a lifelong problem for the mother. Second, that >50 mGy figure sits above the 10 to 35 mGy usually quoted for CT abdomen and pelvis, because these were trauma-protocol pelvic CTs concentrated directly on the fetus rather than dose-optimised modern acquisitions. Both figures are real; quote the one that matches the protocol being discussed.

Practical dose reduction with the C-arm:

• Pulsed rather than continuous screening, at the lowest frame rate that lets you work, and last-image-hold instead of re-screening to look at what you just did.
• Collimate tightly to the field of interest and avoid magnification, which raises dose rate.
• Keep the fetus out of the primary beam wherever the anatomy allows, and remember that scatter falls with the square of distance.
• Position the intensifier close to the patient and the tube far from it; never operate with the tube above the table when it can be below.
• Use a mini C-arm for extremity work — its dose is a fraction of a standard unit and the fetus is nowhere near the beam.
• Brief the radiographer before the incision, and record screening time and dose-area product so a physicist can calculate an actual fetal dose afterwards if anyone asks.
• For non-urgent surgery, the second trimester is conventionally preferred; after about 20 weeks use left lateral tilt to avoid aortocaval compression, which is a circulatory rather than a radiation consideration but belongs in the same plan.

The Pregnant Member of Theatre Staff

The question is asked at least as often about the surgeon, scrub nurse or radiographer as about the patient, and the answer is reassuring. Modelling scattered dose in orthopaedic theatre during fluoroscopically assisted spinal surgery found that even if the annual occupational dose limits were fully exhausted every year for ten years, the resulting risk of congenital malformation would be at least two orders of magnitude below the spontaneous background rate (level 5: phantom simulation with scatter measurement).

Practically: declare the pregnancy so that monitoring applies, wear an additional dosimeter under the lead at waist level, and observe the conventional limit of about 1 mSv to the fetus over the declared remainder of the pregnancy — a figure that routine orthopaedic practice with proper lead and sensible positioning does not come close to. Standing on the intensifier side rather than the tube side, and taking one step back during screening, does more than anything else.

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The reason gestational age matters is that the fetus passes through phases of differing radiosensitivity. Counselling and protocol decisions should reflect this, while remembering that diagnostic doses rarely reach the relevant thresholds.

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Not every musculoskeletal complaint in pregnancy is mechanical. Several pregnancy-associated conditions can mimic trauma or degenerative disease, and recognising them changes both imaging choice and management.

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Global Epidemiology

Trauma complicates roughly 6 to 8 percent of pregnancies and is a leading non-obstetric cause of maternal death, which is precisely why decisive maternal imaging is emphasised worldwide. Musculoskeletal complaints are near-universal in pregnancy: pelvic girdle pain and low back pain affect a large proportion of pregnancies, the majority mechanical and self-limiting, but a minority masking insufficiency fracture, transient osteoporosis, infection or neural compression.

Side-by-Side Guidance

High- vs Limited-Resource Practice

In well-resourced settings, urgent MRI and dose-optimised multidetector CT are usually available around the clock, so the safest adequate test is rarely out of reach. In limited-resource settings, MRI may be unavailable or delayed; here the justified use of radiographs and CT becomes more important, and ultrasound (including FAST in trauma) carries even greater weight as a radiation-free, bedside, low-cost first-line tool. The underlying principle is universal: match the modality to the question, optimise dose, and never withhold imaging that will change maternal management.