A large study of more than 5 million women over 30 years has suggested that CT scans before conception could increase the risk of pregnancy loss and congenital anomalies.
As concerning as the results seem, there are a lot of caveats to consider.
This observational study was conducted in Ontario, Canada, between 1992 and 2023, involving 5,142,339 pregnancies that resulted in 3,451,968 live births. Generally, rates of spontaneous pregnancy loss and congenital anomalies were found to increase in patients who had more CT scans prior to conception.
Compared to patients who had no CT scans, the risk of pregnancy loss increased by 8 percent for those who had 1 scan, 14 percent for 2 scans, and 19 percent for 3 or more scans. The risk of congenital anomalies increased by 6 percent for 1 scan, 11 percent for 2 scans, and 15 percent for 3 or more scans.
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That sounds alarming, but extra context is important.
First, the increase itself is rather small: If, for example, your baseline risk is 10 percent, and it increases by 19 percent after 3 scans, the new risk is 11.9 percent.
Secondly, the study shows a correlation but not necessarily causation, and other factors are almost certainly at play. For instance, people don't tend to require CT scans for trivial reasons – the reason they're getting checked out could be a bigger driver of problems than the scan itself.
"Women who undergo CT scans were more likely to have health conditions – such as diabetes, hypertension, or smoking – that themselves raise pregnancy risks," says Alex Polyakov, gynaecologist at the University of Melbourne in Australia, who was not involved in this study.
"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."
The researchers noted which body parts were imaged in the scan and how that was associated with pregnancy risks. Strangely, it didn't seem to make much difference.
"They find that the increased risk resulting from a head CT scan is pretty similar to the increased risk from a pelvis CT scan," says Derek Hill, biomedical engineer at University College London, who was not involved in the study.
"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."
That lends weight to the idea that the underlying disease is the bigger contributor to the potential risk.
In practice, individual levels of risk should be weighed up appropriately for each patient and reduced where possible. Although the increase in risk is small for any one patient, it's worth prioritizing less harmful alternatives if they can provide similar outcomes.
"These findings support prior efforts to minimize single or repeated CT scans in children and young adults, with greater consideration of the use of ultrasonography and magnetic resonance imaging as first-line methods," the study concludes.
Of course, in many cases, not diagnosing a suspected health problem could have a much greater impact on the risk of pregnancy loss or birth defects. At those times, it would obviously be better to go ahead with the CT scan. This is in line with the current health advice for undergoing CT scans and X-rays during pregnancy.
The research was published in the Annals of Internal Medicine.