The etiologies of this disorder include autoimmune disease, karyotype abnormalities, radiation or chemotherapy, infection, resistant ovary syndrome, ovarian hemorrhage, and idiopathic. Other factors which are associated with an earlier menopause are cigarette smoking and high altitudes. Although the menopause occurs at a specific time, the actual neuroendocrine and ovarian changes leading to the menopause occur over a 5 - 10 year period referred to as the perimenopausal transition.
Follicular attrition begins in utero.
The ovary contains 6 - 7 million oocytes during the 6th month of gestation. There is a further decline to 400,000 oocytes at the time of menarche. It is estimated that there are somewhere between 0 - 10,000 oocytes remaining at the time of menopause. This probably explains why cycles are often anovulatory during the transition. Sex steroid biosynthesis also changes after the menopause.