Some General Rules of When a Set Pattern of Haidh Changes and When Not

1. If the aadat (established and set pattern of haidh) and subsequent change in the aadat are not equal in number of days, the second will be the new aadat.
2. If the two are equal in number, then her number of days remains unchanged, regardless of the number of days being in her usual time, or before, or after, or part of it is in her normal and expected days whilst part is out of it – preceding or following.
3. However, if it corresponds to her aadat in number and time, then there is no change in her aadat at all.
4. If there is a change in number or time then she has a new aadat according to the new number or time.
5. If the bleeding carries on for more than 10 days then she simply follows her previous established cycle (aadat) and there is no change in her aadat.

(Manhalul Waarideen)