Harmless small polyps can also cause bleeding. These are overgrowths of benign tissue, probably owing their existence to estrogen levels that made them grow. Most often they can be gently and painlessly twisted off during a physical exam. If not, they're usually destroyed by the very act of delivering the baby. It's worth getting them off at a checkup so that your doctor won't need to worry and feel obligated to force you into a lot of extra tests you don't need every time there's spotting.
This sounds disastrous, but it usually represents a small clot that causes bleeding and then dissolves away harmlessly. Rarely, the clot dissects between the placenta and the attachment to mother and causes a miscarriage.
Sometimes a small piece of tissue becomes loose and disintegrates through some unknown cause, causing spotting. It's usually a hormonally stimulated collection of menstrual-like tissue that can often be confused with a miscarriage. If it's just tissue debris, it can mean nothing. If it's actual tissue of the pregnancy (fetal or placental), then there should be serious concern, because now this "threatened miscarriage" is re-labeled as "incomplete miscarriage." No one knows why such a phenomenon occurs, but it is harmless. It's the passage of tissue for sure, so it's very disturbing until the pathology report can ease everyone's mind. Strangely enough, the pseudopregnancy of birth control pills can cause this type of tissue shedding, which is really upsetting to a woman who thought (correctly) that she had been using her contraceptive properly. The stabilization of the uterine lining depends on estrogen and progesterone. It's possible that there are lags in the amounts of hormones so that lining not involved with implantation loses its grip and sheds. In my practice, such shedding of only decidual tissue has had no impact on whether a pregnancy will miscarry. The tissue is termed "decidualized," because of the pregnancy-like effect on it at the hands of estrogen.