Loss of fetus
If there is no obvious cause of miscarriage, or aborting a fetus, then it is 'spontaneous'.
Much more common than most think, 20% of all pregnancies. This is important to know, because women often think it is their fault, or feel guilty, or think they won't get pregnant again, or become anxious or depressed. It's not your fault, it is common (most first trimester), doesn't decrease chance next.