Hi OP. I got pregnant in the first cycle after mmc and am now 40+2. I got a +ve pregnancy test about 6 weeks after the ERPC and guessed that I was about 4 weeks at that point, going on the basis of a two week wait since conception plus two weeks that is usually added on when dating based on LMP. I didn't really know how far along I was though until I had the daring scan at 12 weeks although I think the date I was given is a bit early, so am not surprised baby's late. It's all a bit of guesswork if you conceive in the cycle after mc but hopefully it'll be a bit clearer when you go for the dating scan. Have you got a booking appointment with the MW yet?
Not a MMC but I fell pregnant straight after my miscarriage a few years ago. I went to the GP and they referred me for an early scan which dated me at about 7 weeks and was also very reassuring after a miscarriage. HTH.