MRIs in first trimester of pregnancy not associated with increased risk to fetus, study shows

Magnetic resonance imaging is generally thought to be safe for the fetus in the second or third trimesters of pregnancy, but there were no prior controlled studies on its safety in the first trimester, when the fetus forms its major organs and body structures.

To inform clinical guidelines for MRIs on pregnant women, Dr. Joel Ray and colleagues used health data housed at the Institute for Clinical Evaluative Sciences to examine records of more than 1.4 million births in Ontario from 2003 to 2015. They compared women who had first-trimester MRIs with those who had not, and also followed their children up to age 4. The results were published online in the journal JAMA.

"Having an MRI at the earliest stages of pregnancy does not seem to alter the development of the fetus," said Dr. Ray, who is a physician and researcher at St. Michael's Hospital and an adjunct scientist at ICES.

The study also provided new evidence as to how many pregnant women have MRIs. One in every 250 pregnant women in Ontario undergoes an MRI at some point in their pregnancy, including one in every 1,200 having an MRI in the first trimester.

Dr. Ray's team also compared women who had an MRI with the contrast agent gadolinium at any point in pregnancy, with women who did not have an MRI. In this analysis, they did find a higher risk of stillbirth or neonatal death in women who had a gadolinium-enhanced MRI, but the number of events was few. They also found a slightly higher risk of the children developing a rheumatologic or skin condition.

Even though the actual number of stillbirths was low (one in 50), and the outcome of a skin or rheumatological condition was very broadly defined, Dr. Ray said the results support clinical guidelines to avoid giving pregnant women gadolinium unless strongly indicated.

The current study did not include specific information as to why the women received an MRI, or whether they knew they were pregnant at the time. But the study did track the specialty of the physician ordering the MRI: 44 per cent were family physicians, suggesting the MRI may have been booked prior to a woman having conceived. The other common specialty was a neurologist or neurosurgeon, suggesting that some women were investigated for headaches or spinal disc issues.