Prenatal Care is very accurate and doctors and various tests as well as fetal heart pulse are consistently checked...it is very unlikely that if indeed there was a miscarriage at twelve weeks that it would not be dectected on visits.
Dr.'s are pretty accurate,but the first time I was pg,the dr. could not get a heartbeat. They told me you usually can not hear the heartbeat till twelve weeks. So they told me I may have miscalculated how far along I was. I knew I was right and started spotting and decided to see another dr. . Finally after seeing this dr., I found out that the baby's heart had stopped beeting at 11wks.and I was almost 17wks along. The dr. did a d&c. My case may have been very rare, not to say that it happens to everyone, but there is a small chance that it could happen. Best of luck to you!!!