Quick answer
Can I fast while bleeding during Ramadan?
Fasting during ḥayḍ or nifās is not permitted across all four madhhabs. The missed fasts must be made up (qaḍāʾ) after Ramadan ends. Istiḥāḍah, however, does not break the fast.
Fasting during ḥayḍ (menstruation) or nifās (postpartum bleeding) is not permitted. This is the consensus (ijmāʿ) of the four Sunni madhhabs and is established by the ḥadīth in which the Prophet ﷺ said: 'Is it not the case that when she menstruates, she neither prays nor fasts?' (Sahih al-Bukhari)
The fasts missed for ḥayḍ or nifās during Ramadan must be made up (qaḍāʾ) after Ramadan ends, before the next Ramadan begins, if possible. ʿĀʾishah (raḍiyallāhu ʿanhā) said: 'We used to be ordered to make up the fast and we were not ordered to make up the prayer.' (Sahih Muslim) — so prayers missed during ḥayḍ are not made up, only fasts.
If your period begins during the day while fasting, the fast for that day is invalid and must be made up — even if it began moments before maghrib. If your period ends and you perform ghusl during the day, you do not begin fasting that day; you fast the following day and make up the day(s) you missed.
Istiḥāḍah (irregular bleeding outside the recognized ḥayḍ or nifās window) does not break the fast. A woman experiencing istiḥāḍah continues to fast normally, performing wuḍū before each prayer time.
This summary draws on the established positions of the four Sunni madhhabs. For your specific situation, consult a qualified scholar.
Go deeper
Making up missed fasts in Ramadan