Post menstrual bleeding for 12 days discovered today pregnant (day 42) BHCG 290 miu/dl. Is this sure abortion (low BHCG) or threatened abortion?

Threatened abortion. Until the levels are dropping or you are passing tissue it is just a threatened abortion. Miscarriages/ spontaneous abortions usually occur because there is something very serious the matter with the baby and are not due to anything you did or didn't do. Always remember that. Best wishes.
Unsure. This could be either a very early pregnancy or it could be a miscarriage in progress. It would be most helpful to repeat the BhCG about 2 days after the initial check. With a normal pregnancy the BhCG will usually about double every 48 hours. If it is not rising at that rate there is not likely going to be a normal pregnancy.