37 weeks prenatal visit.

I was told at my 37 week appointment today that I haven't been measuring at what I should be and haven't for a few weeks. They want to wait til next week to do a ultrasound. But he has a good heartbeat and she said everything else looked really good. I didn't even think to ask what it meant that he doesn't appear to be growing. Has anyone else had this happen? What do they do about it?