Due date change

Is anyone else dealing with their OB changing their due date? I'm confused because they said they baby is so small that it's a week later then we calculated (which would have put me at just about 5 weeks then) but they were still able to get a heart beat which I thought didn't develop until the 6th week....so I'm not officially confused. (Please note my husband and I were both born 5lbs or less )