Period pains just before Maghrib whilst fasting
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Question: If a woman senses the feeling of blood (in her vagina) but it does not come out until after (the time of) maghrib, or if she feels period pains, is her fast correct for that day or is it obligatory upon her to make it (her fast) up?

Response: If a (non-menstruating) woman senses the feeling of the coming of her menses whilst she is fasting - but no blood comes out until after sunset, or if she feels period pains and no blood comes out until after sunset, then her fast is correct for that day and she is not required to repeat (make up the fast) if it was an obligatory fast, and she is not denied the reward if it was (a) voluntary (fast).

Shaykh ibn ‘Uthaymeen
Fataawa Muhimmah li-Nisaa. al-Ummah - Page 65