Unfortunately, doctors do not always know why a miscarriage may have happened.  Approximately 50% of the time, a miscarriage is the result of a chromosomal abnormality.  Usually, these defects arise spontaneously and are not inherited.  Sometimes a pregnancy loss can occur because of a maternal infection, abdominal or pelvic trauma, a uterine anomaly, or a maternal disease, such as diabetes, lupus, or thyroid dysfunction.  But, often, the answer remains unknown.