Baby measuring small at 36 weeks

Kate
I've been receiving monthly ultrasounds at a perinatal doctor to check for growth of my baby girl, since I have hypothyroidism. All my ultrasounds thus far have been perfect growth-wise but at the one I had yesterday, the doctor told me that baby was measuring small at 4lbs 14oz. I'm 36 weeks today. Dr said they don't know if it's because I am bound to have a small baby (I'm a FTM) or if something else is going on(but is unrelated to my thyroid issues since that's been regulated by an endocrinologist) , but I'm all worried now 😁. Anyone else have small measuring babies??