5w3d pregnant, hcg not doubling as it should. scan showed growth but only heart rate of 66bmp. chances of survival for fetus?

Unsure: so at 5 weeks 3 days you should not even see the pregnancy much less a heartbeat. so it is possible things are just very early and that is why the heart beat is so low. the labs likely are unimportant now. the most prognostic sign is what happens on the ultrasound and whether the pregnancy grow and the heart beat rises

Difficult to know: The heart rate estimation can be a bit off as implied from the M mode and depends on the sonographer getting the optimal angle - however in our area I think our ones are very good - so is the potential may not do so well. May be worth discussion with doctor or LMC re whether serial scans and serial bloods here are helpful, although to be fair if does miscarry at this point unfortunately cant stop