However, in about 50–80% of cases of premature ovarian insufficiency, the cause is unknown, i.e., it is generally idiopathic.An early menopause can be related to cigarette smoking, higher body mass index, racial and ethnic factors, illnesses, and the removal of the uterus.Rates of premature menopause have been found to be significantly higher in fraternal and identical twins; approximately 5% of twins reach menopause before the age of 40. The reasons for this are not completely understood. Transplants of ovarian tissue between identical twins have been successful in restoring fertility. Surgical menopause
Menopause can be surgically induced by bilateral oophorectomy (removal of ovaries), which is often, but not always, done in conjunction with removal of the Fallopian tubes (salpingo-oophorectomy) and uterus (hysterectomy). Cessation of menses as a result of removal of the ovaries is called "surgical menopause". Surgical treatments, such as the removal of ovaries, might cause periods to stop altogether. The sudden and complete drop in hormone levels may produce extreme withdrawal symptoms such as hot flashes, etc. The symptoms of early menopause may be more severe.Removal of the uterus without removal of the ovaries does not directly cause menopause, although pelvic surgery of this type can often precipitate a somewhat earlier menopause, perhaps because of a compromised blood supply to the ovaries. The time between surgery and possible early menopause is due to the fact that ovaries are still producing hormones. Mechanism
The menopausal transition, and postmenopause itself, is a natural change, not usually a disease state or a disorder.
Irregular menstruation is a menstrual disorder whose manifestations include irregular cycle lengths as well as metrorrhagia (vaginal bleeding between expected periods). The possible causes of irregular menstruation may vary. The common factors of it are related to lifestyle, such as stress, body weight, and smoking status. Several studies indicate that COVID-19 vaccine of any type may disrupt the menstrual cycle, although only momentarily. This side effect should resolve on its own in the following month. Irregular cycles or periods
Irregular cycles or irregular periods is an abnormal variation in length of menstrual cycles. An individual usually experiences cycle length variations of up to eight days between the shortest and longest cycle lengths. Lengths ranging between eight and 20 days are considered moderately irregular. Variation of 21 days or more is considered very irregular.Alternatively, a single menstruation cycle may be defined as irregular if it is less than 24 days or more than 38 days. If they are regularly shorter than 21 days or longer than 36 (or 35) days, the condition is termed polymenorrhea or oligomenorrhea, respectively.Additionally, irregular menstruation is common in adolescence. A regular menstrual cycle can be set within a year of menarche. However, other studies suggest that it can take anywhere between 2 and 7 years to establish regularity after an individuals first menses. Other types
Other types of conditions that can be referred to by "irregular menstruation" include:

Metrorrhagia, which generally refers to vaginal bleeding that occurs between the expected menstrual periods. The distinction between irregular cycle lengths and metrorrhagia is not always clear.