If the Woman becomes pure after Fajr, she abstains and makes up for the Fast
Question :
If the woman becomes pure immediately after Fajr time, does she abstain and fast this day with it being considered a day for her, or is it obligatory on her to make up for that day?
Answer:
If the blood stops flowing from her during the time that Fajr comes in or a little before it, her fast is correct and she is credited with fulfullment of that obligation, even if she did not bathe until after it became morning. However, if the blood did not stop until after it was clearly morning (i.e., Fajr time had entered), then she abstains during that day, but it is not accepted from her (as a fast). Rather, she makes up for that day after Ramadhan. And Allâh knows best.
Source:
Fatawa Islamiyah, Vol. 3 Page 297