Scan date vs last period date? Pls see descripcion

I had my first scan and i was supposed to be 7w3d according to my last period (i had very regular cycles) but the scan barely showed a sac with a tiny something, doctor said it looked more like 5 weeks or so… he put me on progesterone and 2 cuantitative HCG to confirm numbers are doubling in the 48h range - first one was 19,800 and just today had 2nd but still no results… so, has anyone had such a big difference between estimated date with last period vs scan and had a successful pregnancy? Could this indicate that it will end in MC? Please comment