Taken from: http://www.medicinenet.com/miscarriage/index.htm

A miscarriage is any pregnancy that ends spontaneously before the fetus can survive. Miscarriage usually occurs before the 13th week of pregnancy. The cause of a miscarriage cannot always be determined. The most common causes of a miscarriage in the first trimester are collagen vascular disease (lupus), hormonal problems, diabetes, chromosomal abnormalities, and congenital abnormalities of the uterus.


http://www.medicinenet.com/miscarriage/article.htm