amniocentesis

i have been having troubles with my blood pressure it started at about 32 weeks pregnant. My doctor has me on 500mg 3 times a day bp meds and it's still staying high. i have no other signs of preeclampsia so he is getting concerned.. i was in the hospital a few nights ago due to high blood pressure and my heart rate dropping..he told me that we will probably have to make a choice soon and deliver earlier than we though (i will be 37 weeks monday and he had me scheduled for a csection at 39 weeks) and he said he might want to do an amnio and i am just wondering if that would be safe and i have read about them and that usually when you're this far along the most that could happen is that you could go into labor? but any advice would be helpful