I agree with Toasted. I've had both abdominal ultrasound (not pregnant, gall bladder issues) and an abdominal ultrasound while pregnant...obviously. lol. With my non-pregnant ultrasounds, they would tell me to take a deep breath and hold it so that they can get a better picture of other organs that aren't readily visable when my lungs aren't full of air. When my lungs are full of air, it pushes the organs down, beneath my ribs so they can get a clear picture of them, and so my ribs aren't in the way. Same thing with a pregnant abdominal ultrasound. Your uterus is growing and pushing things out of the way. She probably wanted a clear view of the top of your uterus, which very well may be underneath your ribcage at this point. So by having you hold your breath, it would push your uterus down, below your ribs, so she can get a clear picture of your uterus without your ribs in the way.