A Canadian study finds more miscarriages in women who had scans before pregnancy

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.

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.