Is it permissible for a woman to pray if she experiences brownish discharge after a week or ten days from her menstrual period due to uterine fibroids, and how should she purify herself for that?

If the menstrual period arrives at its usual time, and it is known with certainty or strong presumption that the last blood is due to an illness, then it is not considered menstruation but rather blood due to an ailment, and one should not abandon prayer or fasting on account of it. Menstruation is blood that exits without a cause, as mentioned by the jurists, such as Sheikh Ahmad al-Dardir, who defined it as blood that exits on its own, not due to childbirth, injury, or an ailment in the body.