All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Muslim jurists stated that when a woman sees the blood of her menstruation at any time during the day of Ramadan her fasting becomes void. Indeed there is a consensus of the scholars about this. Al-Bukhari reported that the Prophet said regarding a menstruating woman: "Is it not that if a woman has a menstruation, she does not pray and does not fast?" The women companions replied: "Yes". So the Prophet responded: "That is an evidence that she has less religions obligations." Therefore, this woman has to break the fast and make it up another day. Anyway, she is rewarded in all cases for fulfilling the commandments of Allaah.
Allaah knows best.