I have read that in menstruation, for example if in one month a woman bleeds for 7 days, then 7 days becomes her habit. in the next month, if she bleeds for 5 days, she must wait another two days before having sexual intercourse to complete her habit of 7 days. Is this correct?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, Sallallahu 'alayhi wa sallam, is His slave and Messenger.
What you have mentioned in the question is not true, because menses may increase [in number of days], and may decrease, and it may come before its regular time and it may be delayed.
Also, bleeding may interrupt and then return, and purity between the two menstrual periods is valid.
So, if the monthly periods of a woman is seven days, for example, and then she sees purity on the fifth day, she should take the initiative to perform Ghusl, pray and fast, and her husband can have sexual intercourse with her, and all the rulings that apply to women who are in purity apply to her. She is not obliged to wait for the time-period of her previous menses; rather, if she does so, she is wrong.
If she again has bleeding, then she is in menses and if she has no bleeding again, then she has become pure from the time she sees purity.
Allah knows best.
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