On fetal MRIs:

"It also does not require intravenous contrast material (dye) or sedation."

http://www.utswmedicine.org/stories/articles/year-2016/fetal...

Fetuses are relatively easier than infants because they are effectively strapped into the womb and don't even know they're getting an MRI. They can obviously wiggle around some, but don't move as much.

Infants are harder:

http://www.medscape.com/viewarticle/499273_11

My daughter had to have a second, sedated MRI because she moved too much during her first one.