Despite many years of study, abnormal chromosome number remains the most common and well-documented cause of miscarriage. Nonchromosomal factors that have been associated with miscarriage are many and include endocrine abnormalities, anatomic abnormalities, inherited and acquired thrombophilia, environmental exposures, immunologic factors, and others. This article attempts to provide a brief overview and critique of the frequently reported factors. In addition, we call attention to the fact that, to be most helpful, modern studies of miscarriage need to provide details about the sonographically determined gestational age and fetal anatomic development prior to or at the time of pregnancy loss. Such information will be critical in helping to sort out which miscarriage-associated factors are more relevant at which stage of fetal development.