She has reached the age of puberty but she is not strong enough to fast

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Check below answers in case you are looking for other related questions:
She thought that the bleeding that results from an abortion was nifaas so she broke her fast
Stopping one’s period during Ramadaan

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Related Answers:
Why is fasting haraam for menstruating women?
If a woman fasts when she is pregnant and is bleeding
Her nifaas (post-partum bleeding) came back when she was fasting
She thought that the bleeding that results from an abortion was nifaas so she broke her fast
Stopping one’s period during Ramadaan

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