Approximately 20% of pregnant women experience some vaginal bleeding, with or without abdominal cramping, during the first trimester. This is known as a threatened abortion. However, most of these pregnancies go on to term with or without treatment. Spontaneous abortion occurs in less than 30% of the women who experience vaginal bleeding during pregnancy.

In the cases that result in spontaneous abortion, the usual cause is fetal death. Such death is typically the result of a chromosomal or developmental abnormality. Other potential causes include infection, maternal anatomic defects, endocrine factors, immunologic factors, and maternal systemic disease.

Estimates report that up to 50% of all fertilized eggs abort spontaneously, usually before the woman knows she is pregnant. Among known pregnancies, the rate is approximately 10%. These usually occur between 7 and 12 weeks of gestation. Increased risk is associated with women over age 35, women with systemic disease (such as diabetes or thyroid dysfunction), and those with a history of 3 or more prior spontaneous abortions.