if you had a scan done at 12 weeks that detected no heart beat then im affraid that baby would have passed away, at 12 weeks if there was going to be a heart beat it would be very clear at that stage,,
your blood tests will say everything is fine even though its not blood tests only measure the hormone level
some people miscarry and still get a positive pregnancy test after they have a d&c becuse the hormone level still remains for some time
the scan will be more accurate then blood tests
sorry to hear your news
i also miscarried at 12 weeks had to have d&c
you must have your d&c asap or you will be at a great risk of infection