Examqo › Examqo library › Radiology › Physics, radiation and safety · IMLE
Imaging in pregnancy and lactation
Below 50 mGy to the fetus no deterministic harm has been described, and most diagnostic examinations are far below that — so a needed examination is not delayed because of pregnancy; US and MRI without gadolinium are preferred, and breastfeeding continues after contrast.
Why it matters in the exam
The exam tests whether the student knows not to fear radiation in the wrong place. Fetal dose depends mainly on the distance between the scanned area and the uterus: a chest radiograph gives the fetus less than 0.01 mGy, a head or chest CT less than 1 mGy, and an abdominal and pelvic CT about 10–25 mGy. The threshold for deterministic harm (malformations, growth restriction, lower IQ) begins at about 100–200 mGy, and the most sensitive weeks for the nervous system are 8–15. In the first two weeks after conception the "all-or-none" principle applies: either loss of the pregnancy or normal continuation.
A classic question is a woman at week 12 with a suspected brain tumour who needs urgent MRI: MRI is done without gadolinium, at any stage of pregnancy, and there is no need to wait for the second trimester. Gadolinium crosses the placenta and remains in the amniotic fluid, so in pregnancy it is given only when the expected information is essential and cannot be obtained otherwise. Intravenous iodinated contrast is allowed in pregnancy when indicated; it crosses the placenta, and the routine newborn thyroid screening test is sufficient.
For suspected pulmonary embolism in pregnancy, start with leg venous Doppler when there are leg symptoms; then choose between CTPA and a perfusion scan — both give a low fetal dose, and CTPA gives a higher dose to the mother's breasts. In lactation: less than 1% of iodinated contrast or gadolinium reaches breast milk, and less than 1% of that is absorbed by the infant's gut — there is no need to stop breastfeeding, and the mother may choose to express and discard milk for 12–24 hours.
Key points
- Below 50 mGy to the fetus — no described deterministic harm.
- CT outside the abdomen gives the fetus less than 1 mGy.
- MRI without gadolinium is allowed in every trimester.
- Gadolinium in pregnancy only when essential.
- Breastfeeding continues after contrast.
Summary table
| Examination or agent | Fetal dose or risk | Allowed in pregnancy? | Breastfeeding |
|---|---|---|---|
| Chest radiograph | Less than 0.01 mGy | Yes, when indicated | No restriction |
| Head or chest CT | Less than 1 mGy | Yes, when indicated | No restriction |
| Abdominal and pelvic CT | About 10–25 mGy | Yes when needed; consider US or MRI first | No restriction |
| MRI without gadolinium | No ionising radiation | Yes, in any trimester | No restriction |
| Gadolinium | Crosses the placenta and remains in amniotic fluid | Only when essential | No need to stop |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Diagnostic radiology, Medical licensing exam (IMLE), Obstetrics and gynecology, Emergency medicine, Nuclear medicine.
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- ACOG Committee Opinion 723: Guidelines for Diagnostic Imaging During Pregnancy and Lactation, 2017, reaffirmed 2023 — Ionising radiation, MRI, contrast agents and lactation
- ACR–SPR Practice Parameter for Imaging Pregnant or Potentially Pregnant Patients with Ionizing Radiation, 2023 — Fetal dose and risk
- ACR Manual on Contrast Media, 2024 — Administration of contrast media to pregnant or potentially pregnant patients; to women who are breast-feeding
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.