Inaccurate pregnancy dates

So I'm considered 10 weeks pregnant. My due date is May 12, 2018. But according to my due date I conceived between August 14-19. Which is inaccurate because I didn't have sex at all those days till the 20s of August. (Not even prior to those days) I still haven't gotten an ultrasound yet. But is it possible that I'm less than 10 weeks and conceived later in August? I usually have long cycles averaging 39 days but sometimes it's less or longer.