Confused on dating pregnancy

Hi ladies. Please may someone help me im totally confused how to work out how many weeks pregnant i am. Doctors go by first day of last menstrual period which was 26th march and this makes me 4 weeks and 5 days today but surely if i conceived 12th april which im certain i did as i tested with ovulation tests this makes me 2wks and 2days. Am i 4wks 5days or 2wks 2days pregnant?? I have read they dont include the first two weeks but does this count if your certain you know your ovulation date? Thanks for any replies its always very appreciated and baby dust to all.