This happened to us at our 20 wk scan LOL! And she actually made us go for a walk and bounce around to wake him up, it worked. She did it mostly because she wanted to see him swallowing fluid to make sure everything was working properly and also to help with the 3D scan. It is stressful at the time maybe if it happens again ask to go for a walk and come back just for your own peace of mind.

*mwa*
Cailin