Expected Date Of Delivery

Hi po. I had my 3rd ultrasound yesterday and based sa result Im on my 35th week of pregnancy and ang due date ko na ay March 10. Pero nung 1st u.sound ko ang nakalagay na due date ay April 1 and on my second u.sound is March 19. Medyo paiba-iba ang due date ko which is nakaka-confuse talaga. My LMP is last June 25 and I am very sure sa monthly period ko. Any idea po kung bakit nagbabago ang EOD?