A miscarriage is the spontaneous loss of a fetus before the 20th week of pregnancy. (Pregnancy losses after the 20th week are called preterm deliveries.)

A miscarriage may also be called a "spontaneous abortion." This refers to naturally occurring events, not medical abortions or surgical abortions.

Other terms for the early loss of pregnancy include:

• Complete abortion: All of the products of conception exit the body
• Incomplete abortion: Only some of the products of conception exit the body
• Inevitable abortion: The symptoms cannot be stopped, and a miscarriage will happen
• Infected abortion: The lining of the womb, or uterus, and any remaining products of conception become infected
• Missed abortion: The pregnancy is lost and the products of conception do not exit the body

See also: Threatened miscarriage

Most miscarriages are caused by chromosome problems that make it impossible for the baby to develop. Usually, these problems are unrelated to the mother or father's genes.

Other possible causes for miscarriage include:

• Hormone problems
• Infection
• Physical problems with the mother's reproductive organs
• Problem with the body's immune response
• Serious body-wide ( systemic) diseases in the mother (such as uncontrolled diabetes)

It is estimated that up to half of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among those women who know they are pregnant, the miscarriage rate is about 15-20%. Most miscarriages occur during the first 7 weeks of pregnancy. The rate of miscarriage drops after the baby's heart beat is detected.

The risk for miscarriage is higher in women:

• Older than 35
• Who have had previous miscarriages