In case you're interested, it's here. A more recent article from a different clinic gives the predictive power of seeing a heartbeat at slightly lower - 82% - but I'm not clear whether they are all idiopathic recurrent miscarriers, or whether they include some with clotting disorders, and likewise it's not clear from the abstract whether they are using "heartrate over X" or just "presence of heartbeat" as the diagnostic factor. Perhaps someone with a subscription (Thalia?) can enlighten.
(Later - I really shouldn't have carried on reading as I just found another article saying the predicitive power is only 71% in over-40s. Excuse me while I go and sit in a hole).