"Miscarriage" is the lay term for the natural or accidental termination of a pregnancy at a stage where the fetus is incapable of surviving. The medical term for it is "abortion"; when the abortion is not deliberately induced, it is termed a "spontaneous abortion," so that is a synonym for "miscarriage." Miscarriages can occur for many reasons, not all of which can be identified. A miscarriage can result from maternal exposure to certain chemicals, diseases, or physical trauma (including x-rays or insect bites as well as falls or blows), or it may result from biological defects in the mother or in the developing fetus.
Most such terminations occur very early in pregnancy, during the first trimester, and many people restrict the term "miscarriage" to terminations during the second trimester. Miscarriages frequently occur so early that the woman is not even aware that she is pregnant. Some women are prone to miscarry; the term "habitual abortion" describes the condition where three consecutive pregnancies have terminated before 20 weeks gestation.