LMP vs Late Ultrasound dd

According to my LMP I am 31 weeks today. My midwive never had me done a early ultrasound instead had me done a 5th mth scan which puts me at 29 weeks 4 days. I know ultrasound done this late the dates can be inaccurate. I know when had my last period also but doctor is going by late scan date. I would like to hear everyone opinion on this and late scanning. For me I'm going with my LMP.