No growth the last three weeks

I went to see my OB today, and when he did my measurements he told me that I haven't grown in the last three weeks ( from 37-40) so they are sending me for a growth ultrasound on Friday to ensure baby is growing well and fluid levels are still ok. I'm due tomorrow. Has anyone else experienced this? He didn't seem alarmed but this is our miracle and rainbow baby, and after all we have been through its hard to not feel upset or concerned!