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

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.