If a woman began the day with a valid fast but menstruation starts at noon or later, that day’s fast ceases to be valid and is made up subsequently. Its invalidation does not depend on a deliberate act of eating.
This is a natural and legally recognized impediment, not the type of deliberate Ramadan violation that triggers the special expiation.
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Kitab al-Sawm: Unconsciousness, Recovery, Menstruation and Abstention
If a woman began the day with a valid fast but menstruation starts at noon or later, that day’s fast ceases to be valid and is made up subsequently. Its invalidation does not depend on a deliberate act of eating.
This is a natural and legally recognized impediment, not the type of deliberate Ramadan violation that triggers the special expiation.