Miscarriage is one of the most frequent problems in human pregnancy. The most widely accepted definition is that proposed by the World Health Organization (WHO) in 1977. The incidence among clinical pregnancies is about 12-15% but including early pregnancy losses it is 17-22%. The only two etiologic factors recognized by all authors are uterine malformations and parental balanced chromosomal rearrangements. There have been many other suggested risk factors. In this revision we discuss these.