Pregnant women should avoid having magnetic resonance imaging (MRI) unless absolutely necessary, because risk to the developing fetus is unknown. Experts especially urge pregnant women not to have an MRI during the first trimester (12 weeks) of their pregnancy. As a result, ultrasound (which uses sound waves to create an image) is more likely to be used to create images for women in the first trimester of pregnancy.

Women who are breastfeeding should consult a physician or radiologist about whether it is appropriate to undergo an MRI. In some cases, a woman may pump breast milk before the procedure and refrain from breastfeeding until the contrast material has cleared her body. The radiologist or technician will be able to provide information as to when it is safe to resume breastfeeding following an MRI that uses a contrast medium.

Patients with tattoos or permanent makeup are encouraged to consult with their physician before an MRI is performed. These patients might feel some mild discomfort or a burning feeling on their skin due to a reaction between iron oxide or other metallic substances commonly found in the darker inks of the tattoo and the magnetic field generated during the test. Furthermore, large or very dark tattoos can also cause “artifacts,” or false shadows to appear on the film produced from the test.

If the MRI involves injection of a contrast medium, there is the possibility of a reaction. Patients should inform their physician and the technician if they have an allergic reaction to shellfish. The contrast medium contains iodine, the same element found in many types of shellfish. Patients with an allergic reaction may experience nausea, headache or pain at the site of injection. In severe cases, the patient may develop an anaphalactic  reaction with swelling in the mucous membranes, such as in the throat, resulting in the inability to breathe.