It is prohibited (ḥarām) for a menstruating woman or a woman in postpartum bleeding (nifās) to observe any type of fast. However, they are required to make up (qaḍā’) the fasts they miss at a later time. Even if bleeding begins shortly before sunset while a woman is fasting, that day’s fast is invalidated and must also be made up.
ʿĀ’ishah (may Allah be pleased with her) narrates:
“When we experienced menstruation during the time of the Messenger of Allah (peace and blessings be upon him), we were instructed to make up the missed fasts, but we were not instructed to make up the prayers.”
(Muslim, Ḥayḍ, 69)