I had this problem too. I had to wait for my first ultrasound to calculate the due date. The measurements of the embryo gave me a due date of Aug 1.
I think if your Dr. uses the date of your m/c then your dates could be off a bit. If my Dr. used that method then my due date would be in early July which is not correct.
I was told by the doctor I saw after my MC that it's the reason they don't like you to get pregnant before AF returns because it creates dating problems. Anyhow, I don't see why they can't do a quick US, even vaginal to figure it out. The waiting is awful.
If you know the day you ovulated, then you subtract 7 days and then add 9 months for the due date.
I know the day, give or take a day or two before that. I used 11/28 and just typed it into a pregnancy calendar. It gives me the date of 8/21/07, so that is the date I'm using. But my BhCG was kind of high going by that date. Maybe twins! Who knows...
Yayy,Twins! My DD is Aug.17(ish).
Trust your dates! Add 2 weeks to the day you think you got pregnant (as if that was your last AF) and that is how far you are. That is how I did it and it turned out to be spot on. Apparently, they cannot be accurate with the dates so early even if they do an u/s, I didn't have a dating scan until I was 10 weeks and although my midwife thought I had conceived 2 weeks after the mc I knew it had been 3 weeks after and I was right!