Shall a woman break her fast if she is in her period, or shall she fast?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.

Referring to your question, menstruation and puerperium do prevent a woman from fasting according to most Muslim scholars. She also must make up the same number of missing days.

It is narrated that the messenger of Allah said: “O womenfolk! You should give charity and ask much forgiveness for I saw you in bulk amongst the dwellers of Hell. A wise lady among them said: “Why is it, Messenger of Allah, that our folk is in bulk in Hell?” Upon this the prophet observed: “you curse too much and are ungrateful to your spouses. I have seen lacking in common sense and failing in religion, but (at the same time) robbing the wisdom of the wise, besides you. Upon this, the woman remarked: What is wrong with our common sense and with religion? He observed: Your lack of common sense (can be well judged from the fact) the evidence of two women is equal to one man (in financial contracts in specific), that is a proof of common sense, and you spend some nights (and days) in which you do not offer prayer and in the month of Ramadan (during the days) you do not observe fast, that is a failing in religion.” Related by Muslim.

The Palestinian Legal Verdict Council decided that menstruation and puerperium do break the fast, and thus, women are asked to compensate those days.