Growth ultrasound?

Going by my lmp I'm about 36.5 weeks which is what my drs have me at. My first ultrasound would put me at 37+1 which I've been going by but the last (elective) scan I had at 28 weeks put me at 38 weeks. During my office visit today she said I measured at 38 but said ge didnt feel like a big baby. Then she said I will do a growth ultrasound next week because I'm labeled "gestational diabetic".

Do they always do a growth scan for diabetes, do they do it if you aren't too? I was under the impression my last scan from my office would be 20 weeks.