Radiation in pregnancy: fetal dose and counseling
Table of contents
A 29-year-old teacher, about 11 weeks pregnant after conception, arrives at the emergency department after being knocked down by the crowd fleeing an explosion with radioactive material in a square. She has left flank pain, her blood pressure is 104 over 66 mmHg and her heart rate is 104 beats per minute. The detector shows mild contamination on her hair and hands. Crying, she asks whether the baby will be born with a problem and whether she should end the pregnancy. The scene is fictional and was written for teaching.
This text uses the scene as a clinical reasoning scenario for students and health professionals. The topic is radiation exposure in pregnancy: how to estimate the fetal dose, what the known thresholds are for each phase and how to counsel without increasing fear. The text does not discuss cancer radiotherapy in pregnancy or radiation in space. Decontamination and triage of many victims are in the post about dirty bomb, triage and decontamination. If you play the case of the day on NEXO, you will recognize the challenge: fear of the CT scan can cost the mother her life and, with her, the life of the fetus.
Key points
- The fetal outcome depends on stabilizing the mother, and the CT scan should not be denied out of fear of radiation.
- According to the CDC, non-cancer effects are not detectable with a fetal dose below about 0.1 Gy, or 100 mGy, at any phase.
- The risk depends on the dose and the gestational age; between the 8th and 15th week after conception, the brain is more vulnerable.
- Estimating the fetal dose should involve medical physics and consider external and internal sources.
- Radioactive iodine crosses the placenta, and the fetal thyroid begins to take it up around the 10th to 12th week.
Why this is a reasoning scenario
The scene brings together three fears: fear of the explosion, fear of radiation and fear of harming the baby. A review on radiation in pregnancy notes that an exaggerated perception of radiation harm can increase the number of terminations of wanted pregnancies, and that teratology counseling is a simple and inexpensive service to relieve anxiety and avoid unnecessary terminations after diagnostic tests (Seven et al., 2022). The post about diagnostic uncertainty helps with the conversation when the dose is still unknown.
Radiation exposure in pregnancy is more common than it seems. A 2023 review cites a multicenter study with more than 3 million pregnancies in which CT use rose from 2 to 9.3 per 1000 pregnancies over 20 years, and in which 5.3 percent of pregnant women had some exam with ionizing radiation (Mainprize et al., 2023). The same review argues for recommendations that reduce anxiety, reassure those with excessive concern and avoid unjustified terminations linked to diagnostic tests (Mainprize et al., 2023).
What the scene teaches
The scene trains six habits. The first is to treat the mother first. The second is not to deny the necessary exam. The third is to estimate the fetal dose with help from medical physics. The fourth is to read the thresholds by gestational age. The fifth is to remember internal contamination and the fetal thyroid. The sixth is to counsel with honest numbers.
What is not at stake
The text does not discuss doses of medicines to remove radionuclides in a pregnant woman, which depend on the package insert and the specialist, nor the decision to end the pregnancy, which belongs to the woman and involves factors other than radiation. The post about signs, symptoms and history helps date the pregnancy accurately, which changes how the risk is read.
The mother first
Stabilizing the mother
A review of the image of the pregnant patient in the trauma room states that, in mild or severe trauma, the fetus outcome depends on stabilizing the mother, which should be the focus of initial management (MacDermott et al., 2024). If CT is indicated as in any other trauma patient, there should be no hesitation, and the exam should not be denied for fear of radiation, because the risk of missing a severe injury in the mother outweighs any potential risk to the fetus (MacDermott et al., 2024). The post about clinical deterioration recalls that tachycardia in the pregnant patient can hide significant bleeding.
In a French multicenter study with polytraumatized pregnant patients, the median dose received by the fetus in the initial assessment was 23 mGy (Abback et al., 2023). In the same study, only 31 percent of professionals knew that the probability of fetal harm increases above a threshold of 100 mGy (Abback et al., 2023). The study recalls that trauma during pregnancy can cause miscarriage, preterm birth, growth restriction and fetal death, and that a cumulative dose below 100 mGy carries very little risk at any stage of pregnancy (Abback et al., 2023).
Reducing the dose without delaying the diagnosis
A review of dose reduction strategies in the emergency department recalls that ultrasound and MRI have advantages over CT in the pregnant patient, but that, in many emergency departments, CT remains the exam of choice (Picone et al., 2023). Every exam involving radiation should follow the principle of keeping the dose as low as reasonably possible (MacDermott et al., 2024). On scene, ultrasound in the emergency room shows free fluid, and CT of the abdomen confirms a splenic injury, treated with embolization. The post about clinical decision-making helps choose the sequence of exams.
Estimating the fetal dose
The sources of the dose
The CDC recalls that the fetal dose tends to be lower than the maternal dose, because of the protection of the uterus and surrounding tissues (CDC, radiation and pregnancy for clinicians). The estimate must consider the dose from an external source in the mother's abdomen, the dose from substances inhaled or ingested that reach the blood and can cross the placenta, and the dose from substances that concentrate in maternal tissues near the uterus, such as the bladder (CDC, radiation and pregnancy for clinicians). Medical physicists and radiation protection specialists at the hospital are good resources for estimating the fetal dose (CDC, radiation and pregnancy for clinicians).
The CDC cites as references NCRP report 174, on exposure before conception and during pregnancy, and ICRP publication 84, on pregnancy and medical radiation (CDC, radiation and pregnancy for clinicians). In the scene, the hospital's medical physicist estimates the fetal dose from the CT and concludes that, added to the exposure in the square, it is well below 100 mGy. The post about interprofessional clinical reasoning recalls that this calculation is team work.
How much each exam delivers
The review on dose reduction states that diagnostic fluoroscopy and CT of the abdomen and pelvis deliver higher doses than other exams, but that the estimated threshold for spontaneous abortion, of 50 to 100 mGy or more, is higher than a single CT should reach (Picone et al., 2023). The Swedish review on imaging in pregnancy recalls that short-term deterministic effects are not expected below 100 mGy, which is important for X-ray and nuclear medicine exams (Mattsson et al., 2021).
The thresholds by gestational age
The CDC table
The CDC summarizes the non-cancer effects of acute fetal exposure in a table adapted from NCRP report 174, which serves as a guide and not as a definitive recommendation (CDC, radiation and pregnancy for clinicians). The table below summarizes the main points, with time counted from conception.
| Acute dose to the fetus | Up to 2 weeks | 3rd to 5th week | 6th to 13th week | 14th to 23rd week | 24th week to term |
|---|---|---|---|---|---|
| Below 0.1 Gy | No detectable non-cancer effects | Not detectable | Not detectable | Not detectable | Not detectable |
| 0.1 to 0.5 Gy | Implantation failure may increase slightly | Growth restriction possible | Growth restriction possible | Unlikely effects | Unlikely effects |
| Above 0.5 Gy | Implantation failure likely high | Miscarriage may increase; severe malformations and growth restriction more likely | Miscarriage may increase; growth restriction likely | Miscarriage may increase; growth restriction possible | Miscarriage and neonatal death may occur |
Table source: (CDC, radiation and pregnancy for clinicians). In the first 2 weeks after conception, the concern with a dose of 0.1 Gy or more is death of the embryo; embryos that survive are unlikely to have congenital anomalies, whatever the dose (CDC, radiation and pregnancy for clinicians). One review calls this phase the all-or-nothing period (Seven and colleagues, 2022).
The numbers vary between sources
Not all sources use the same thresholds. The review on imaging in the trauma room cites 50 to 100 mGy for implantation failure in the first 2 weeks, more than 200 mGy for malformations between the 3rd and 8th week and more than 100 mGy for IQ loss between the 8th and 15th week (MacDermott and colleagues, 2024). It adds that several guidelines conclude that the risk to the fetus is negligible below 50 mGy (MacDermott and colleagues, 2024). The Turkish review cites the NCRP considering up to 50 mGy negligible and recalls that anomalies and miscarriages tend to increase from 100 to 150 mGy (Seven and colleagues, 2022). The post on differential diagnosis helps deal with sources that do not match.
The common message is that isolated diagnostic exams stay below these thresholds, and that the real concern starts with accident or intentional exposure doses above regulatory limits (CDC, radiation and pregnancy for clinicians). The CDC states that most forms of exposure during pregnancy, such as diagnostic exams or occupational exposure within limits, probably do not cause effects on the fetus (CDC, radiation and pregnancy for clinicians).
The teacher in the scene
At about 11 weeks after conception and with an estimated dose well below 0.1 Gy, the teacher is in the range where the CDC does not expect detectable non-cancer effects. The conversation with her can start there, with the numbers, and not with the list of possible risks at much higher doses.
The developing brain and cancer risk
The brain between the 8th and 15th week
According to the CDC, the most vulnerable period for intellectual disability is between the 8th and 15th week after conception, with severe disability possible above 0.5 Gy (CDC, radiation and pregnancy for clinicians). The prevalence of IQ below 70 was 40 percent after 1 Gy between the 8th and 15th week and 15 percent after 1 Gy between the 16th and 25th week (CDC, radiation and pregnancy for clinicians). These data come from atomic bomb survivors exposed in utero.
A review on brain defects after prenatal exposure summarizes that, in these survivors, there was an increase in intellectual disability, low IQ, worse school performance and seizures with no apparent cause (Verreet and colleagues, 2016). The effects were roughly proportional to dose, with a 43 percent increase in the risk of intellectual disability and a drop of 25 to 29 IQ points per Gy (Verreet and colleagues, 2016). The Swedish review adds that all observations of major IQ drop and severe disability refer to fetal doses of 500 mGy or more, at high dose rates (Mattsson and colleagues, 2021).
Small head and growth
In the cohort exposed in utero to the atomic bombs, the frequency of small head showed a dose response (Sugiyama and colleagues, 2021). In a study of newborns exposed in utero to Chernobyl fallout, the fetal thyroid dose of iodine-131 ranged from 0 to 2263 mGy, with a mean of 61 mGy, and the reduction in head circumference with dose was greater in those exposed in the first trimester, although the interaction with trimester was not statistically significant (Hatch and colleagues, 2017).
The risk of cancer
The CDC notes that exposure of the embryo or fetus can increase the risk of cancer in the children, especially above 0.1 Gy, well above the typical doses of diagnostic radiology (CDC, radiation and pregnancy for clinicians). From the survivor data, the theoretical risk of cancer in adult life up to age 50 was about 10 times higher in those exposed in early childhood than in those exposed before birth, and there are no reliable data on which stage of pregnancy is most sensitive (CDC, radiation and pregnancy for clinicians).
The Swedish review estimates the increase in childhood cancer risk at about 1 in 10,000 per mGy after exposure in the womb, and notes that a fetal exposure of 10 to 20 mGy can increase the risk of leukemia by 1.5 to 2 times over a baseline risk of about 1 in 3000 (Mattsson and colleagues, 2021). The review on dose reduction cites a lifetime cancer risk of about 0.4 percent per 10 mGy of fetal dose (Picone and colleagues, 2023). A study on chromosomal aberrations in those exposed in the womb notes that the association between irradiation in the womb and later cancer is still debated, partly because of biases and the small size of the studies (Cologne and colleagues, 2022).
In the mortality cohort of those exposed in the womb to the atomic bombs, deaths from noncancer diseases between 4 and 39 years of age were higher than expected in those exposed to 0.6 Gy or more, but the authors note that the interpretation of these effects remains uncertain (Sugiyama and colleagues, 2021). The post on diagnostic pause helps avoid turning a small risk into a certainty in the conversation with the pregnant woman.
Internal contamination and the fetal thyroid
What crosses the placenta
The CDC notes that most radioactive substances that reach the mother's blood can be detected in the fetal blood, and that some, such as iodine, concentrate more in the fetus than in the corresponding maternal tissue (CDC, radiation and pregnancy for clinicians). The dose in specific fetal organs matters for substances that localize there: iodine-131 and iodine-123 in the thyroid, iron-59 in the liver, gallium-67 in the spleen and strontium-90 and yttrium-90 in the skeleton (CDC, radiation and pregnancy for clinicians).
The fetal thyroid
A study of those exposed in the womb to iodine-131 from Chernobyl explains that the placenta is freely permeable to iodines and that, with the onset of thyroid function around the 10th to 12th week of pregnancy, the fetal thyroid begins to accumulate iodine from maternal blood (Neta and colleagues, 2014). For this reason, the iodine-131 dose in the fetal thyroid is minimal early in pregnancy and maximal in the third trimester (Neta and colleagues, 2014). The description of the Belarus cohort adds that the very small size of the gland and the high cell proliferation can make the fetal thyroid especially sensitive (Yauseyenka and colleagues, 2020).
In this Belarus cohort, the authors cite the Ukrainian study that found a significant increase in the risk of thyroid nodules related to iodine-131 and an increase, without statistical significance, in the risk of thyroid cancer (Yauseyenka and colleagues, 2020). In the study of adolescents exposed in the womb, there was no overall association between the iodine-131 dose in the thyroid and height, weight or body mass index (Neta and colleagues, 2014).
The teacher's contamination
In the scene, the contamination is external and mild, on the hair and hands, and comes off with a shower and washing. The radiation protection team takes a nasal swab and requests urine to rule out internal contamination. If the material were radioactive iodine, the gestational age of 11 weeks would lead the team to think about the fetal thyroid and to discuss with the specialist the protection of the maternal and fetal thyroid. The post on simulation of care helps train the conversation with the pregnant woman.
The counseling
The CDC notes that informing about radiation in pregnancy is part of preventive prenatal care, and that the goal is always to promote the health and quality of life of the pregnant woman and the child (CDC, radiation and pregnancy for clinicians). In the Turkish review, teratology counseling served to relieve anxiety and avoid unnecessary interruptions after diagnostic exams (Seven and colleagues, 2022). The review also notes that, under normal conditions, the fetus can receive up to 5 mGy during pregnancy (Seven and colleagues, 2022).
The outcome of the scene
The teacher receives in writing the estimated dose, the gestational age and the explanation that, in this range, the CDC does not expect detectable non-cancer effects. Prenatal care continues with routine morphological ultrasound. The decision about the pregnancy remains hers, now with numbers instead of fear.
How NEXO helps train this reasoning
In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by searching for the ICD-10 code. The pregnant woman exposed to radiation is a good exercise because it forces you to weigh two patients and to talk about risk honestly. The post about case-based learning in clinical practice explains why this format fixes knowledge.
Three exercises with cases
Change the gestational age. Say how you would read the same scenario if the teacher were 1 week after conception.
Change the dose. Say what would change if medical physics estimated 0.7 Gy to the fetus at 11 weeks.
Change the material. Say what you would investigate if the source were iodine-131 and the pregnancy were at 30 weeks.
The post about acute radiation syndrome helps when the mother's dose is also high.
The limits on using the game are described on the NEXO economy page. If you research teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information on curation and creators, and institutions can learn about the proposal on the partnerships page.
Frequently asked questions
Can I perform a CT scan on a traumatized pregnant woman?
Yes, if it is indicated. The risk of missing a serious injury in the mother outweighs the risk to the fetus (MacDermott and colleagues, 2024).
Below what dose are malformations not expected?
According to the CDC, non-cancer effects are not detectable below about 0.1 Gy at any stage (CDC, radiation and pregnancy for clinicians).
Which is the most sensitive phase for the brain?
Between the 8th and 15th week after conception (CDC, radiation and pregnancy for clinicians).
Does one CT scan justify terminating the pregnancy?
A single CT scan should not reach the threshold estimated for spontaneous abortion (Picone and colleagues, 2023); counseling helps avoid unnecessary interruptions (Seven and colleagues, 2022).
Does radioactive iodine reach the fetus?
Yes. The placenta is permeable to iodines, and the fetal thyroid takes up iodine from the 10th to 12th week (Neta and colleagues, 2014).
Final checklist
- I stabilized the mother first of all.
- I did not deny the necessary exam out of fear of radiation.
- I dated the pregnancy in weeks after conception.
- I asked medical physics for the estimated fetal dose.
- I considered external sources and internal contamination.
- I read the risk in the table by dose and gestational age.
- I thought about the fetal thyroid if there was radioactive iodine.
- I counseled with numbers and in writing.
- I arranged prenatal follow-up.
Want to practice reasoning with progressive clues? Play the case of the day.
References
- Abback PS, Benchetrit A, Delhaye N, et al. Multiple trauma in pregnant women: injury assessment, fetal radiation exposure and mortality. A multicentre observational study. Scand J Trauma Resusc Emerg Med. 2023. PMID 37131266.
- Centers for Disease Control and Prevention. Radiation and Pregnancy: Information for Clinicians. Page dated 01/31/2025, accessed on 10/24/2026. Official source.
- Cologne J, Sugiyama H, Hamasaki K, et al. Chromosome aberrations among atomic-bomb survivors exposed in utero: updated analysis accounting for revised radiation doses and smoking. Radiat Environ Biophys. 2022. PMID 35175360.
- Hatch M, Little MP, Brenner AV, et al. Neonatal outcomes following exposure in utero to fallout from Chernobyl. Eur J Epidemiol. 2017. PMID 28856527.
- MacDermott R, Berger FH, Phillips A, et al. Initial Imaging of Pregnant Patients in the Trauma Bay-Discussion and Review of Presentations at a Level-1 Trauma Centre. Diagnostics (Basel). 2024. PMID 38337792.
- Mainprize JG, Yaffe MJ, Chawla T, et al. Effects of ionizing radiation exposure during pregnancy. Abdom Radiol (NY). 2023. PMID 36933026.
- Mattsson S, Leide-Svegborn S, Andersson M. X-RAY AND MOLECULAR IMAGING DURING PREGNANCY AND BREASTFEEDING-WHEN SHOULD WE BE WORRIED? Radiat Prot Dosimetry. 2021. PMID 33855370.
- Neta G, Hatch M, Kitahara CM, et al. In utero exposure to iodine-131 from Chernobyl fallout and anthropometric characteristics in adolescence. Radiat Res. 2014. PMID 24611659.
- Picone C, Fusco R, Tonerini M, et al. Dose Reduction Strategies for Pregnant Women in Emergency Settings. J Clin Med. 2023. PMID 36902633.
- Seven M, Yigin AK, Agirbasli D, et al. Radiation exposure in pregnancy: outcomes, perceptions and teratological counseling in Turkish women. Ann Saudi Med. 2022. PMID 35229664.
- Sugiyama H, Misumi M, Sakata R, et al. Mortality among individuals exposed to atomic bomb radiation in utero: 1950-2012. Eur J Epidemiol. 2021. PMID 33492551.
- Verreet T, Verslegers M, Quintens R, et al. Current Evidence for Developmental, Structural, and Functional Brain Defects following Prenatal Radiation Exposure. Neural Plast. 2016. PMID 27382490.
- Yauseyenka V, Drozdovitch V, Ostroumova E, et al. Belarusian in utero cohort: A new opportunity to evaluate the health effects of prenatal and early-life exposure to ionising radiation. J Radiol Prot. 2020. PMID 31770737.