Dr changed my due date due to..please help?

i am being monitored for high blood pressure for a month now and at my last appt i had my second nst which was beautiful but my blood pressure is still high so finally after a month they put me on a low dose of bp meds. However my doctor mentioned that i wouldnt see my due date becuase my placenta will get tired before then and he said that i would probably deliver a few weeks early but not after the 8/9 my due date was 8/16 is it right for him to make that call with out an ultrasound for development. i due have one on 7/11 but i feel he is jumping the gun..and yes i know full term is 37wks but now im just nervous being 33 or 34wks that it could happen n e time...opinions pleaseeeeeeeee