Date of delivery

Hello po everyone.. Im 33 weeks pregnant and my OB told me that starting April 20 mghahanda na po ako for delivery but based on my ultrasound MAY 8 pa yung expected date ko.. And nagtataka lng po tlaga ako kac hndi ko tlga alam yung last menstruation ko kac i had my PCOS with me very irregular period ko.. For now parang my mga signs na ako na malapit na ako manganganak.. Any help po??