Accepting what a woman says about the beginning and end of a period
Question

                If a man divorces his wife, and he does not know that she is menstruating, then after the divorce the woman says that she was menstruating at the time of the divorce, should what she says about that be accepted?.
Praise be to Allah.
    Firstly: 

    In the answer to question no.
    72417 we have stated that the 
    scholars differed as to whether the divorce of a menstruating woman counts 
    as valid, but the correct view is that it does not count. 

    Secondly: 

    The word of a woman concerning the start and end of her 
    period, and other matters of which men have no knowledge, is to be accepted 
    because she is to be trusted with that. Al-Shaafa’i (may Allaah have mercy 
    on him) said: “Sufyaan narrated to us from ‘Amr ibn Dinar that ‘Ubayd ibn 
    ‘Umayr said: A woman is to be trusted with regard to personal matters.” End 
    quote from al-Umm, 5/225. 

    The fact that a woman’s words concerning such matters is to 
    be accepted is indicated by the verse in which Allaah says (interpretation 
    of the meaning): 

    “and it is not lawful for them to conceal what Allaah has 
    created in their wombs, if they believe in Allaah and the Last Day”

    [al-Baqarah 2:228]
al-Jassaas (may Allaah 
    have mercy on him) said: The fact that Allaah commands women not to conceal 
    (what He has created in their wombs) indicates that what a woman says about 
    menstruation or the absence thereof is to be accepted, and the same applies 
    to pregnancy too, because these are both things that Allaah has created in 
    her womb. If a woman’s word concerning these matters were not acceptable, 
    then He would not have warned woman not to conceal them. Thus it is 
    established that if a woman says, “I am menstruating”, it is not permissible 
    for her husband to have intercourse with her, and when she says, “My period 
    is over,” it is permissible for him to have intercourse with her. By the 
    same token our companions said that if he says to her, “You are divorced 
    when you get your period,” and she says, “I have got my period,” then she is 
    divorced and her words are like proof. End quote from Ahkaam al-Qur’aan, 
    1/506 

    Al-Sa’di (may Allaah have mercy on him) said (p. 102): 

    This indicates that what a woman says about matters that 
    concern her and of which no one else can have any knowledge, such as 
    pregnancy, menstruation, etc., is to be accepted. End quote. 

    It says in Mu’een al-Hukkaam  (p. 95): Concerning 
    passing judgement on the basis of the statement of one woman in matters of 
    which women alone have knowledge: that is matters concerning which only 
    women have knowledge, such as childbirth, whether a woman is a virgin or 
    not, menstruation, pregnancy, miscarriage, faults of free woman and slave 
    woman and everything that is beneath their garments. The point here is that 
    because these are matters to which men are not privy and they have no 
    knowledge of them, then in these cases the testimony of women is regarded as 
    equal to that of men, because of necessity. End quote. 

    In order for a woman’s words concerning these matters to be 
    accepted, her claim that a period has begun or ended must come at a time 
    when that is possible; if she makes such a claim at a time when it is not 
    possible, then her words should not be accepted. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

    Among the things that we learn from this verse are the 
    following: 

    What a woman says concerning her ‘iddah is to be accepted, 
    because Allaah , may He be exalted, says: “and it is not lawful for them 
    to conceal what Allaah has created in their wombs” [al-Baqarah 2:228]. 
    The point here is that Allaah has made their words carry weight; if they did 
    not carry weight, then their concealing (what has been created in their 
    wombs) would not have any effect. So if she says that her ‘iddah has ended, 
    and that is within the possible time frame, then she should be believed, and 
    she is to be trusted with regard to that. But if she claims that her ‘iddah 
    has ended within an impossible time frame, then her words are to be 
    rejected, because one of the conditions of a claim being accepted is that it 
    should be something that is possible, and a claim of something that is 
    impossible should not be listened to at all. End quote.  

    Tafseer Soorat al-Baqarah, p. 
    102 

    See also: al-Mughni, 7/158; al-Fawaakih al-Dawaani, 
    2/34 

    And Allaah knows best.