What is the ruling of the Shari'ah regarding the waiting period ('iddah) for a divorced woman if her menstruation is irregular or has ceased for a long time before reaching the age of menopause?

The default 'iddah (definition) (waiting period) for a divorced woman who menstruates is three menstrual cycles, based on the Quranic verse: {And divorced women shall wait concerning themselves for three menstrual periods.}

As for the woman whose menstruation has ceased without a known cause, her 'iddah is nine months for potential pregnancy, and then she adds another three months, like the 'iddah of a woman who has reached menopause, making her waiting period a full year. If menstruation returns during this year, she restarts her 'iddah based on menstruation. This is the view preferred by the Malikis and Hanbalis, and it is the opinion of Umar ibn al-Khattab.

Ibn Qudamah said: "If she menstruates once or twice and then her menstruation ceases, and she does not know why it ceased, her 'iddah does not end until a year after the cessation of menstruation, as ruled by Umar ibn al-Khattab."

There is no impediment to the marriage of a woman whose menstruation has ceased before the age of menopause.