A cohort study published in Annals of Internal Medicine looks at CT scans before pregnancy and birth defects and miscarriage risk.
Prof Simon Jolly, Professor of Visceral Physics, University College London (UCL), said:
“The outcome of this study has important implications. As the editorial acknowledges, the carcinogenic potential of ionising radiation — from medical imaging or other natural sources — is well known. The carcinogenic risk of CT scans is generally considered to be insignificant compared to the benefit provided by the diagnostic images.
“However, the study suggests that there are increased risks from CT scans that have impacts beyond cancer. It highlights that the increased risk of pregnancy loss for patients receiving pre-conception CT scans — so, importantly, not just during pregnancy — is significant enough that other non-ionising imaging modalities, such as MRI and ultrasound, may need to be prioritised for women of child-bearing age or planning to conceive.
“It is important that this is investigated in more detail and either correlated or refuted in other patient cohorts. It should be noted that the statistical significance and fraction of total pregnancies means that there is an absolute as well as relative risk increase for pregnancies. It should also be noted that, whilst the impact of direct irradiation of the abdomen and pelvis results in the highest risk, there is still increased risk from CT scans to other parts of the anatomy which is worthy of further investigation.”
Prof Derek Hill, Professor of Medical Imaging Science, UCL, said:
“This large study from Canada involves data from more than 5 million pregnant women. It finds that women who had an x-ray CT scan in the month prior to conception had a modest increase in chance of a failed pregnancy (rising from about 10% to nearly 13% for a single CT scan) compared to women who had no CT scan. This risk rises if there were multiple CT scans in that month. The authors argue that in young women, alternative types of imaging to CT should be considered.
“While any failed pregnancy is hugely distressing, it is important not to over-interpret the results of this paper. Firstly, the use of potentially harmful ionising radiation (such as in a CT scan) is already based on the principle of keeping any dose as low as reasonably achievable. Hospitals are very aware of the risks of excess radiation exposure. Therefore, CT scans are used when there is a clear benefit to the patient that outweighs the risk. There usually isn’t a radiation-free alternative available that provides the same quality of information: if there were it would have been used instead. It is important to note the statement by the authors that “The findings should not hinder the use of CT imaging when indicated, given the widespread availability of and rapid access to CT technology and the quality of CT imaging for many conditions “.
“More fundamentally, this paper shows a link between CT scan in the month before conception, and the risk of failed pregnancy. It does not demonstrate that the radiation from the CT scan causes this increased risk. It seems quite likely that women who have a CT scan in the month before conception already have some sort of health problem that led to the need for a CT scan in the first place. And if they had multiple CT scans, the health problem was probably more serious. This same health problem may have increased their risk of failed pregnancy. The results in the paper do indeed point to this likelihood. The authors have an internal control in the paper: as consider which part of the body was imaged in the CT scan. They find that the increased risk resulting from a head CT scan is pretty similar to the increased risk from a pelvis CT scan. Yet a head CT scan gives a much lower dose to the reproductive organs than a pelvis CT scan. If the increased risk of pregnancy were due to radiation dose, we would have expected the pelvis CT scan to lead to a much bigger risk than a head CT scan. It therefore seems likely that at least some of the increased risk of failed pregnancy associated with a CT scan in the month before conception is a result of the health condition that led to the CT scan in the first place, rather than to the radiation dose from the CT scan.”
Comments below were gathered by the Australian SMC:
Associate Professor Alex Polyakov, Clinical Associate Professor at the Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne & Medical Director of Genea Fertility Melbourne, said:
“This impressive retrospective Canadian study, involving over five million pregnancies, examined whether CT scans carried out before conception might influence later pregnancy outcomes. The results showed a slight increase in miscarriage and congenital anomalies among women who had undergone CT scans before conceiving.
“At first glance, this sounds alarming. But the size of the increase was modest. For example, miscarriage occurred in about 10% of pregnancies without CT exposure, compared with 11.7% after a single scan. Such differences are statistically detectable in an extensive dataset, but may not be meaningful at the individual patient level. Another challenge is separating the effects of the scan itself from the reasons for having one. Women who undergo CT scans were more likely to have health conditions—such as diabetes, hypertension, or smoking—that themselves raise pregnancy risks. Also, a woman scanned for trauma, suspected cancer or serious medical condition may already have had an elevated baseline risk prior to a CT scan.
“Although the researchers attempted to adjust for these factors, some degree of ‘confounding’ almost certainly remains. Association does not equal causation: just because one event follows another does not mean the first causes the second. The study relied on health-care databases, which cannot capture every miscarriage, anomaly or patient characteristic. This means the absolute risks are likely to be somewhat imprecise.
“Overall, the findings should not discourage appropriate CT imaging. The study does not prove that CT scans before pregnancy cause harm. Still, it reinforces the principle of caution: use CT when necessary, but preference non-radiation producing alternatives, such as ultrasound or MRI, where they can provide the same answers. The issue of accessibility and cost would need to be considered, as CT scans are more widely available and are likely to be more cost-effective compared to other imaging modalities.”
Prof Gavin Pereira, Epidemiologist, Curtin University, said:
“The Ontario cohort study reports a modest dose response between CT before pregnancy and risks of miscarriage and congenital anomalies. The main concern is confounding by indication, since many scans are prompted by conditions that themselves raise miscarriage risk, including major trauma, acute abdominal or pelvic emergencies and severe infection, none of which are fully captured in routine data.
“Because the analysis includes only recognised pregnancies, collider bias is also possible: if CT lowers fecundability and lower fecundability is linked to miscarriage, conditioning on conception selects more resilient exposed women and attenuates the observed effect. These opposing biases act in different directions, so the true effect size remains uncertain.
“Replication should use designs that strengthen causal inference. Clinicians should continue to interpret the associations cautiously and, where clinically reasonable, prefer non ionising imaging.”
‘Exposure to Computed Tomography Before Pregnancy and Risk for Pregnancy Loss and Congenital Anomalies’ by Simard et al. was published in Annals of Internal Medicine at 22:00 UK time on Monday 8th September 2025.
DOI: 10.7326/ANNALS-24-03479
Declared interests
Prof Simon Jolly: “I have not received industry funding for any of my research. I have received in-kind contributions from industrial collaborators in the Czech Republic (Nuvia) in the form of sheets of plastic scintillator material, but they work in the related field of health physics and background radiation monitoring, not in medical diagnostic imaging.
“I am otherwise solely funded for my research by UK government grants and have previously received an EU H2020 grant.”
Prof Derek Hill: “Derek is co-founder and director of Panoramic Digital Health, but this commercial relationship is unrelated to CT scanning and ionising radiation.”
Dr Alex Polyakov: None
Prof Gavin Pereira: None