A spontaneous abortion is the loss of a fetus during pregnancy due to natural causes. The term "miscarriage" is the spontaneous termination of a pregnancy before fetal development has reached 20 weeks. Pregnancy losses after the 20th week are categorized as preterm deliveries.

The term "spontaneous abortion" refers to these naturally occurring events, not elective or therapeutic abortion procedures.

More specific terms include: missed abortion (a pregnancy demise where nothing is expelled); incomplete abortion (not all of the products of conception are expelled); complete abortion (all of the products of conception are expelled); threatened abortion (symptoms indicate a miscarriage is possible); inevitable abortion (the symptoms cannot be stopped, and a miscarriage will happen); and infected abortion.

The cause of most spontaneous abortions is fetal death due to fetal genetic abnormalities, usually unrelated to the mother. Other possible causes for spontaneous abortion include: infection, physical problems the mother may have, hormone (endocrine) factors, immune responses, and serious systemic diseases of the mother (such as diabetes or thyroid problems).

It is estimated that up to 50% of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among known pregnancies, the rate of spontaneous abortion is approximately 10% and usually occurs between the 7th and 12th weeks of pregnancy.

The risk for spontaneous abortion is higher in women over age 35, in women with systemic disease (such as diabetes or thyroid dysfunction), and women with a history of three or more prior spontaneous abortions.