CF - i had my BT at what we thought was 15DPO - HCG came back at 26. implantation can occur anywhere from 6-12DPO, so if you have implanted late, it's normal that you'd have a low HCG to start. and if you're "guesstimating" O date, that makes it even more "normal"

FWIW, my levels went from 26 to 270 5 days later and then up to approx 3000 a week later, so doubling every 36hours -ish.

the start number isn't the important bit, it's the pattern of it climbing that is more important