What is the ruling on a menstruating woman fasting without knowing the conditions for a valid fast, and what is the expiation for that?

The fasting of a menstruating woman is not valid and not permissible, and she is obligated to make up for it. If she fasts out of ignorance of the prohibition, there is no sin upon her if she is a new convert to Islam or lives in an environment where ignorance is widespread. However, if she lives in an environment where its people are not ignorant of that, she may be sinful due to her negligence in seeking the obligatory knowledge, and she must repent. In all cases, expiation (kaffarah) is not required of her.