I had a blighted ovum, and I found at at 9wks, the sac measured about 6.5 wks. I waited 2wks to miscarry naturally, it didn't happen, so at almost 12wks I had a d&c. I think it depends on the person and their body.
My mom had a pregnancy a few months after I was born (of course this was before u/s). Anyhow, she knew something was wrong with the pregnancy, but did not want to have a d&c, just in case. She carried her pregnancy for 5mo before she miscarried.
Huge hugs, I know how you feel. Sometimes life doesn't make much sense. I knew at 3 weeks (we found out REALLY early) that things weren't going well w/ my pg. At 6 weeks we learned there was no hb and at seven wks the numbers started to drop rapidly (I had betas drawn every couple of days and they had slowly been going up - which is why we knew something was wrong) My Doc suggested that if things didn't happen naturally by 8 weeks he suggested a d&C. Two days later the D&C was no longer an issue.
I'm sorry that you are struggling. I know that after almost a year there are days when I still wished I understood the whys and werefores! GL