Hi, and thanks for your medical question on eHealth forum
You can know that something is wrong when you are pregnant, when they do the ultrasounds. Also the routine ante natal tests can detect abnormalities.
Bleeding from vagina too heralds an abortion. In advanced pregnancy loss of fetal movement is a bad sign. Abdominal cramps and low back ache before term too is not good and can lead to abortion. Hope this helps. Take care!