Miscarriage is the most common complication of pregnancy. It is defined as a loss of a pregnancy before 24 weeks of gestation. There are various terms used to describe this event, including miscarriage, spontaneous abortion, missed abortion, early pregnancy loss, early pregnancy failure, blighted ovum, and nonviable pregnancy. There is no consensus in the literature, and miscarriage is the term used in this chapter to describe first-trimester pregnancy loss.