In the critical first trimester of pregnancy, undergoing magnetic resonance imaging (MRI) without a contrast agent is not associated with any negative outcomes for the baby, according to a new study.

But when the expectant mother has an MRI with gadolinium contrast, a material that makes it easier to see internal structures like blood vessels in the brain, risk for stillbirth, neonatal death or the child having any of several serious health conditions may rise, researchers say.

"We know that MRI is actually being encouraged as the main high fidelity method of imaging in women of reproductive age," since it does not involve radiation, and produces outstanding images of deeper cavities that ultrasound can't get at, said lead author Dr. Joel Ray of St. Michael's Hospital and the Institute for Clinical Evaluative Sciences in Toronto.

The new results "invigorate or support" what most people believed by intuition or smaller datasets: that non-contrast MRI is generally safe for pregnant women, Ray told Reuters Health.

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With contrast MRIs, negative outcomes are still very rare but it may be a good idea to avoid these MRIs in pregnancy, or when a woman may be pregnant, he said.

The researchers analyzed more than 1 million births between 2003 and 2013 in Ontario, Canada, using universal healthcare databases. About one in 250 women had undergone an MRI during pregnancy, one in 1,200 during the first trimester and one in 3,000 with gadolinium contrast. Health of the babies was followed from birth until they were 4 years old.

Having an MRI during the first trimester was not tied to an increased risk of stillbirth, newborn death, cancer, congenital defects or hearing loss in the children.

Only 397 women had gadolinium contrast MRIs during pregnancy, compared to more than 1.4 million without any MRI in this comparison.

Connective tissue or infiltrating skin conditions like nephrogenic systemic fibrosis were about 36 percent more common in babies born to the gadolinium group than the no-MRI group, according to the results in JAMA. More broadly, rheumatological and inflammatory conditions were similarly increased by about 45 cases per 1,000 children per year.

There were seven stillbirths or newborn deaths in the gadolinium group compared to 9,844 in the no-MRI group, so while still rare, these outcomes were twice as common in the gadolinium group, the authors note.

An adult body processes and clears gadolinium contrast materials through the urine, but a fetus exposed via the placenta will continually urinate and ingest the same material in the amniotic fluid until birth, Ray said.

"The theory was that you can recirculate gadolinium over and over again, and this brings that theory a little bit closer to a clinical point," he said.

Gadolinium can usually be avoided during pregnancy, but "in a woman who is pregnant, if she really needs gadolinium then she's the priority," he said.

"Potential risk versus benefit should always be assessed when an imaging study is recommended," said Dr. Dorothy Bulas, professor of pediatrics and radiology at George Washington University School of Medicine and Health Sciences in Washington, D.C. "This study is reassuring and helps support the use of MRI at any gestation if there is a valid indication for the exam," she said by email.

"The study also supports the need to avoid gadolinium in the pregnant patient unless the maternal benefits truly outweigh the potential fetal risks that were noted in this large study cohort," Bulas added.

Current contrast media guidelines already incorporate appropriate safety concerns, said Dr. Karen Oh of Oregon Health and Sciences University in Portland, who was not part of the new study.

"Doctors should continue to provide appropriate prenatal care and utilize MRI when indicated; and women should ask and receive explanations for indications for studies during pregnancy, have their questions answered appropriately, so that they can feel comfortable with the care they are receiving," Oh told Reuters Health by email.