If a woman's menstruation ceases due to illness, her waiting period is three months.

The Shaykh : If a woman used to menstruate, but her menstruation ceased due to an illness, and she recovered from the illness yet the menstruation did not return, we look at the situation. If she says, "May Allah have mercy on you," and the doctors state that the menstruation cannot return due to a defect in the uterus, she is treated like a postmenopausal woman and observes a waiting period of three months. However, if there is hope that the menstruation will return, she waits until it returns, and then she observes the waiting period based on that.