Answered by Shaykh Amjad Rasheed
If a woman begins her day fasting while she is pure, and then during the day, her menstruation begins, does she finish off the fast?
Menstruation and post-natal bleeding both break the fast when they begin. Both also negate worship, so it is haram for a menstruating woman to hold back from what breaks the fast [s. such as food] with the intention to fast.
However, if she holds back from what breaks the fast without the intention to fast, that is permissible for her, for it is not obligatory on her to act like someone who is not fasting, as has come in the Tuhfa [Ibn Hajar’s commentary on Nawawi’s Minhaj]. (3-414)
Amjad Rasheed
[Translated by Sr. Shazia Ahmad]