Is a miscarriage imminent if my hcg is not doubling?

No: Serum HCG levels are expected to rise by 2/3 (not double) every 48 hours or so in the first 8 weeks of pregnancy. However all doctors have had a patient where the HCG rose , just not by that much and ended up in a normal pregnancy. An HCG level that doesn't rise at all or falls is much more concerning for miscarriage.

Not necessarily.: As with anything else in nature, there is a physiologic variation regarding the doubling time for beta HCG in the first trimester. The rule of thumb is that it should double every 48 hours if it is a viable intrauterine pregnancy. Failure to do so suggests blighted ovum/miscarriage, ectopic pregnancy, or even normal early pregnancy. See your gynecologist and get pelvic ultrasound to know for sure!