oligomenorrhea

...Menopause before age 40 is called early menopause, and menopause after age 55 is called late menopause.
[Menstrual cycle abnormalities]
Abnormal menstrual cycle refers to a period that is abnormally long or short outside the normal range of 25 to 35 days in mature women. The former is called oligomenorrhea and the latter polymenorrhea, and is often due to ovarian insufficiency. (1) Oligomenorrhea When the menstrual cycle is extended to 39 to 40 days or more and for more than three months, it becomes secondary amenorrhea.

*Some of the terminology explanations that mention "oligomenorrhea" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…40歳以前の閉経を早期閉経，56歳以上の閉経を遅発閉経という。
［月経周期の異常］
　月経周期の異常とは，成熟婦人における月経周期日数25～35日の正常範囲をはずれて，異常に長く延長したり，また異常に短い場合で，前者を希発月経oligomenorrhea，後者を頻発月経polymenorrheaといい，卵巣機能の不全に基づくことが多い。(1)希発月経　月経周期が39～40日以上に延長する場合で，3ヵ月以上の場合は続発性無月経になる。…

※「oligomenorrhea」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報