About how many women have pregnant signs, then miscarry, but didn't realize they were pregnant?

Less than 1% While it certainly possible to miscarry prior to confirmation of pregnancy, I do not receive many queries or meet many patients who have had that experience. Without a positve pregnancy test, there is no way to know with certainty that there ever was a pregnancy. I occasionally see a patient who had a positive home test who is negative at the office, who then knew they were pg b4 they miscarried.
Not many. Today many women do home tests in these circumstances thes tests are so accurate that not to know is rare.