Answered by Shaykh Amjad Rasheed
Is it permissible for a menstruating woman to make the intention to fast the next day of Ramadan, it being probable that her period will end and she can then fast?
If a menstruating woman, before her period is over, makes the intention to fast the next day of Ramadan, it is valid, on the condition that she has already completed the days of her menstrual habit and that according to her habit, her period should end that night.
However, if the days of her habit have not been completed or if her habit is different, then her intention is not valid, because her intention is not based on something valid as the erudite scholar Ibn Hajar said in the Tuhfa (3:397)
And all this is valid only if her period stops before dawn, for if it continues until after dawn, even if a little, her fast is not valid at all.
Amjad Rasheed
[Translated by Sr. Shazia Ahmad]