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.
A woman who should break her fast because of menses during the month of Ramadan should do so only when she sees the blood of menses but not before, therefore, if she does not see the blood during the time of her usual menses period, then she is obliged to fast. The fact that the time of the menses changes does not affect the fasting.
Finally, it should be noted that the blood which the woman sees fifteen days after her menses is considered menses as clarified in Fatwa 91444.
For more benefit on the minimum period of purity, please refer to Fatwa 91497, and for the maximum period of menses, please refer to Fatwa 91605.
Allaah Knows best.