Imaging in pregnancy: what is safe
- Choosing a study
Ultrasound and MRI use no radiation; most X-rays and CTs give the baby a small dose, and a needed study should not be withheld.
What it is
Imaging a pregnant woman balances two risks: the small risk of radiation or contrast to the fetus, and the larger risk of missing a serious diagnosis in the mother, such as pulmonary embolism, appendicitis or trauma. Ultrasound and MRI without contrast use no ionising radiation and come first when they can answer the question. Studies that do not include the abdomen and pelvis, such as a chest X-ray, a head CT or a CT pulmonary angiogram, expose the fetus mainly to scatter and give very small fetal doses. Even CT of the abdomen and pelvis usually stays well below the level at which harm to the fetus has been seen (around 50 to 100 mGy), so a needed study should be done, with the dose kept as low as reasonable. Ask about pregnancy and test before pelvic studies in women of reproductive age.
How to decide
- First ask: can ultrasound or MRI without contrast answer the question? If yes, use them.
- Fetal dose depends mostly on distance from the uterus: chest, head and limb studies give a tiny fetal dose; abdominal and pelvic CT gives the most.
- Harmful effects such as malformation, growth restriction or reduced IQ have been seen only above roughly 50 to 100 mGy, more than nearly any single diagnostic study.
- Timing: in the first two weeks after conception, exposure tends to cause loss or nothing («all or none»); organogenesis and early brain development (about weeks 8 to 15) are the most sensitive periods.
- A small increase in childhood cancer risk is assumed at any dose, which is why dose is kept as low as reasonable, not why a needed test is refused.
- For comparison, natural background radiation gives about 3 mSv a year; a chest X-ray is equivalent to a few days of it.
| Study | Safe in pregnancy? | Notes |
|---|---|---|
| Ultrasound (including Doppler) | Yes | No ionising radiation; first choice for the pelvis, abdomen, veins and the fetus. |
| MRI without contrast | Yes, when indicated | No radiation; useful for appendicitis, placenta accreta and the brain; done in any trimester if it changes management. |
| MRI with gadolinium contrast | Avoid unless essential | Gadolinium crosses the placenta and stays in the amniotic fluid; give only if the benefit clearly outweighs the risk. |
| Chest, limb or head X-ray | Yes | Fetal dose is tiny (mostly scatter); do not delay for pregnancy. |
| Abdominal or pelvic X-ray | Yes, if needed | Fetal dose is low but higher than chest films; ultrasound may answer the question instead. |
| CT of head, neck or chest, including CT pulmonary angiogram | Yes, if indicated | Very low fetal dose; the mother's breast receives more dose than the fetus. |
| CT of abdomen and pelvis | Yes, if needed | Highest fetal dose of the common studies but usually well below the harm threshold; use when ultrasound and MRI cannot answer or are not available. |
| Iodinated contrast for CT | Yes, if needed | Crosses the placenta; the newborn's thyroid function is checked after birth (routine neonatal screening). |
| Ventilation–perfusion scan | Yes | Low fetal dose; an option for pulmonary embolism when the chest X-ray is normal. |
| Mammography | Yes, if needed | Very low fetal dose; ultrasound is the first test for a breast lump in pregnancy. |
| Radioactive iodine treatment | No | Contraindicated: the fetal thyroid takes up iodine from about the end of the first trimester. |
Common mistakes
- Withholding a CT pulmonary angiogram or an abdominal CT because of pregnancy, when it is the right test, puts both mother and fetus at greater risk.
- Forgetting to ask about pregnancy before pelvic or abdominal studies in women of reproductive age; when unsure, do a pregnancy test.
- Treating a study done before pregnancy was known as a reason to end the pregnancy: the fetal dose of almost all diagnostic studies is far too low to justify that.
- Lead shielding over the abdomen adds little for studies outside the pelvis, because most fetal exposure is internal scatter, and it can hide anatomy or raise the dose.
- Telling a breastfeeding mother to stop after iodinated or gadolinium contrast: only a tiny amount reaches the milk, and current guidance allows breastfeeding to continue.
Next step
- Order the study that answers the clinical question, write «pregnant, about N weeks» on the request and ask whether ultrasound or MRI could replace it.
- Suspected appendicitis in pregnancy: ultrasound first; if inconclusive, MRI without contrast; CT if MRI is not available quickly.
- Suspected pulmonary embolism in pregnancy: leg-vein ultrasound if there are leg symptoms, chest X-ray, then CT pulmonary angiogram or V/Q scan by local protocol.
- Exposure already happened (pregnancy discovered later): ask a medical physicist or the radiologist to estimate the fetal dose and counsel the woman with that number.
- Gadolinium, nuclear medicine or interventional procedures in pregnancy: discuss with the radiologist before booking.
Test yourself
Think of the answer, then tap the question to see it.
Above roughly what fetal dose have deterministic effects such as malformations been seen?
Around 50 to 100 mGy, a level beyond nearly every single diagnostic study, including most abdominal and pelvic CT scans.
Why is a CT pulmonary angiogram acceptable in pregnancy?
The uterus is outside the beam, so the fetus receives only a very small scatter dose; missing a pulmonary embolism is the greater danger.
What is checked in the newborn after iodinated contrast was given during pregnancy?
Thyroid function, because iodine crosses the placenta; routine neonatal screening covers it.
Why is gadolinium avoided in pregnancy unless essential?
It crosses the placenta and lingers in the amniotic fluid, where the fetus keeps swallowing it, and its long-term fetal safety is not established.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Radiation Exposure In Pregnancy (StatPearls (NCBI Bookshelf), 2023); Radiation Safety in Medical Imaging (StatPearls (NCBI Bookshelf), 2023); Intravenous Contrast Media in Medical Imaging (StatPearls (NCBI Bookshelf), 2023).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.