Uy, mama, please don't wait until week 26, go see your OB or go to the ER na if the headache is really that severe, lalo na at 39 weeks ka na! Headaches like that, especially heavy pressure sa likod ng tenga, need to be checked right away para sa peace of mind mo.
A few of us mums had similar concerns near the end, at ang OB talaga ang best person to rule out anything. Alam mo naman, better safe than sorry tayo especially ngayon na malapit na ang iyong due date. Hang in there, at sana mabilis na macheck ka!