I've had both done as a real deal, depending on what sort of MRI it is be prepared to lay down and be as still as possible for a bit. I had a renal MRI and it was a long process cause they iject you full of iodine to see the veings and all that, the first time the guy didnt take the pic right and i had to sit around and then go back in a hour later so they could try again and get the right shot.
The ultrasound is nothing, just a bunch of goop though, ask for a printut to see your baby (heart

)
Neither required any prep, if they inject you with ANYTHING during the MRI go get a liter of water or 2 and get a buncha water in your system otherwise it can make ya a bit sickly
