Haven't been to the doctors yet but trying to figure it out myself, my period is due tomorrow my LMP was sept 11th, no known date of ovulation but I think I ovulate later than 14 days after. My cycles are 36 days long. So based off my LMP my due date is around June 18th but if you tie in the fact my cycles are longer it is saying June 27th. That's a bit of a difference, what one do the doctors go by?