Measuring behind at first scan

I had a scan today due to some spotting yesterday. I got my bfp on the 19th of December, bloods was 79 on the 24th and 1224 on the 30th. I got positive opks the 9th,10th and 11th of December. Fetal Pole was seen but no HB yet and sac was measuring 5.1 weeks which is 5weeks 1 or 2 days by their calculations. I should be 5weeks5days- 6weeks?

They’ve rebooked me a scan for the 20th and I’ve moved my private scan forward to next Monday. They said if there’s no progress in 2 weeks then we’ll talk about my options which makes me think that it’s going to be bad news, especially since I’ve got a bleed on the outside of the babies sac.

Anyone else measure behind which made it seem like you got a bfp at like 4dpo and everything was ok?