The causes of primary ovarian insuficiency (POI) include chromosomal and genetic defects, autoimmune processes, chemotherapy, radiation, infections and surgery, but many are unidentified (idiopathic). The age of 40 is set by convention but is supported by clinical observations. From a statistical point of view, the age limit of 40 is approximately two standard deviations (SD) below the average age at natural menopause (50 plus/minus 4 years).