Stopped performing her salaah believing her menses had started
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Question: A woman began to bleed and continued to do so for nine days, so she stopped performing her salaah because (she thought) it was her menses. After afew days her real menses started, so what should she do? Should she make up the salaah she missed on those days (when the blood was not that of the menses) or not?

Response: That which is better is for her to make up the salaah which she missed in the first days (prior to her real menses beginning), and if she does not do so, then there is no harm in that. This is because the Prophet (sal-Allaahu `alayhe wa sallam) did not require this of a menstruating woman who said she suffers from severe menses (for longer than the average time) and thereby leaves her salaah (during the entire period).

So the Prophet (sal-Allaahu `alayhe wa sallam) ordered her to allow six or seven days for her menses and then to perform her salaah for the rest of the (days of the) month, and he did not order her to make up the (salaah) which she had left.

So if she makes up the salaah which she left, then that is good, and if she does not, then nothing is required of her (i.e. there is no harm in that).

Shaykh Ibn 'Uthaymeen
Fataawa al-Haydh wal-Istihaadhah wan-Nifaas - Page 69, No.54