The Difference in ‘Iddah (Waiting Period) according to the Different Circumstances of the Women
19-11-2020 | IslamWeb
Question:
What is the Eddath period for menopausal women
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
If a woman is observing ‘Iddah for the death of her husband, then menstruation is irrelevant. Rather, she is required to observe ‘Iddah of four months and ten days, as evidenced by the Saying of Allah, The Exalted, (what means): {And those who are taken in death among you and leave wives behind – they, [the wives, shall] wait four months and ten [days]...} [Quran 2:234]
If she is observing her ‘Iddah after a divorce and she is menopausal (ceased to menstruate due to reaching the age of menopause), her ‘Iddah would be three months because Allah, The Exalted, Says (what means): {And those who no longer expect menstruation among your women – if you doubt, then their ‘Iddah is three months, and [also for] those who have not menstruated.} [Quran 65:4]
If she is not menopausal but does not experience menstruation for some reason, she is required to observe ‘Iddah for a full year. If a year elapses and she does not experience menstruation, marriage becomes lawful to her. If she experiences menstruation during that year, she is required to observe her ‘Iddah according to her menstrual cycle, i.e. three menstrual periods, or wait for another year, and so on, until she experiences three menstrual periods. Ibn Qudaamah 
said in Al-Mughni: "In summary, if a man divorces his wife, who experiences menstruation, but she does not get her regular menstrual period after the divorce and does not know the reason, she should observe her ‘Iddah for a year."
He also said: "If she experiences one or two menstrual periods but then does not experience menstruation and does not know why she ceased to menstruate, her ‘Iddah ends after one year from the cessation of menstruation. This is evidenced by the report narrated on the authority of ‘Umar 
when he said about a man who divorced his wife and she experienced one or two menstrual periods and then did not experience menstruation and did not know why she ceased to menstruate: ‘She has to wait for nine months; if she is proven non-pregnant, she should observe a ‘Iddah of three additional months, and that is a total of one year.’ We do not know of any opinion opposing to that. Ibn Al-Munthir said: ''Umar judged among the Muhaajireen and the Ansaar according to this rule, and no one disproved it.'" [End of quote]
Allah Knows best.