Because Allah, the Blessed and Exalted, has said (in Surah Al-Baqarah) that it is not lawful for them to conceal what Allah has created in their wombs. (Therefore, just as this statement is to be accepted, likewise their statement regarding menstruation will also be accepted.)

And it is narrated from Ali (may Allah be pleased with him) and Qadi Shurayh that if a woman’s family member, who is religious, testifies that this woman menstruates three times in a month, then his testimony will be accepted. And Ata bin Abi Rabah said that the days of a woman’s menstruation will be accepted as many as they used to be before (according to her previous habit), i.e., before divorce and so on. Ibrahim Nakha’i has also said the same. And Ata said that the minimum duration of menstruation is one day and the maximum is fifteen days. Mu’tamir narrates from his father Sulaiman that he asked Ibn Sirin about a woman who, after five days of menstruation according to her habit, sees blood again, so he replied:“Women know more about this.”

Narrated `Aisha: Fatima bint Abi Hubaish asked the Prophet, "I got persistent bleeding (in between the periods) and do not become clean. Shall I give up prayers?" He replied, "No, this is from a blood vessel. Give up the prayers only for the days on which you usually get the menses and then take a bath and offer your prayers."