She committed zina with a Christian, then she married a Muslim – what is the ruling on the children they produce?
Question

                A Muslim girl lived with a Christian man without any marriage contract for a period of time, then she left him and married a Muslim. What is the ruling on what she did? 

Is she permissible for her Muslim husband? What is the ruling on the children that she has with her Muslim husband? Peace be on those who follow true guidance.
Praise be to Allah.
    This woman has to repent to 
    Allaah from the sins and evil actions that she has committed, regret that 
    deeply, resolve never to do it again, and do as many righteous deeds as she 
    can, because Allaah says (interpretation of the meaning): 

    “And verily, I am indeed forgiving to him who repents, 
    believes (in My Oneness, and associates none in worship with Me) and does 
    righteous good deeds, and then remains constant in doing them (till his 
    death)”

    [Ta-Ha 20:82]

    1 – The Islamic ruling on 
    that is clear: zina is an immoral action and a major sin, and a 
    reprehensible crime for which a person deserves the curse, anger and wrath 
    of Allaah for transgressing His sacred limits; and deserves His punishment 
    in this world by being given one hundred lashes if he or she is not 
    previously-married, and being stoned to death if he or she was 
    previously-married. We ask Allaah to keep us safe and sound. 

    But whoever commits any such 
    action then repents sincerely to Allaah, his bad deeds will be turned to 
    good deeds. Allaah says (interpretation of the meaning): 

    “And those who invoke not any other ilaah (god) along with 
    Allaah, nor kill such person as Allaah has forbidden, except for just cause, 
    nor commit illegal sexual intercourse  
    and whoever does this shall receive the punishment.

    69. The torment will be 
    doubled to him on the Day of Resurrection, and he will abide therein in 
    disgrace;

    70. Except those who 
    repent and believe (in Islamic Monotheism), and do righteous deeds; for 
    those, Allaah will change their sins into good deeds, and Allaah is 
    Oft-Forgiving, Most Merciful”

    [al-Furqaan 25:68-70]

    2 – With regard to her 
    marriage to a Muslim man, if it was done after she repented and established 
    that she was not pregnant as a result of zina, then there is nothing wrong 
    with that, otherwise it is haraam, because it is not permissible to marry a 
    woman who is known to have committed zina until after she has repented and 
    established that she is not pregnant by waiting one menstrual cycle, 
    according to the most correct scholarly view. The Hanbalis and Maalikis are 
    of the view that she should establish she is not pregnant by waiting three 
    menstrual cycles, as in the case of a woman who is divorced by talaaq. The 
    former view was narrated from Ahmad (may Allaah have mercy on him) and was 
    favoured by Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him), and 
    is supported by the report which speaks of establishing that a female 
    prisoner of war (i.e., a woman captured by the Muslim army in a war against 
    the kuffaar) is not pregnant, which was narrated from Abu Sa’eed al-Khudri 
    (may Allaah be pleased with him) who said that the Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said concerning the female 
    prisoners of Awtaas: “No one should have intercourse with a pregnant 
    (concubine) until she has given birth, or with one who is not pregnant until 
    one menstrual cycle has passed.” Classed as saheeh by al-Albaani in 
    Saheeh Abi Dawood. 

    The benefit of establishing 
    that she is not pregnant by waiting one menstrual cycle is to ensure that 
    she is not pregnant as a result of zina, so that the “water” (semen) of 
    marriage will not be mixed with that of an immoral relationship. 

    If he married her during the 
    time when it had not yet been established whether she was pregnant or not, 
    then the marriage should be annulled until that has been established, after 
    which he may marry her if he wants. 

    3 – With regard to the ruling 
    on the children, doubts apply only to the first pregnancy after the 
    marriage. If she gave birth to a child after less than six months from the 
    date of the marriage, then according to sharee’ah the child cannot be 
    attributed to the husband or named after him, because that falls short of 
    the minimum length of pregnancy which is six months. 

    If she gave birth six months 
    or more from the date of the marriage, then the child should be attributed 
    to the husband and is regarded as his son, even if she married without 
    having repented or establishing that she was not pregnant, so the child 
    should be named after the husband in this case. 

    See al-Mughni, 7/108;
    al-Fataawa al-Kubra by Ibn Taymiyah, 3/176; Mawaahib al-Jaleel, 
    3/413

    It should also be noted that 
    the basic principle concerning the phrase “Peace be upon those who follow 
    true guidance” is that it should be said to a kaafir when writing to him, 
    not to a Muslim. 

    It says in Kishshaaf 
    al-Qinaa’ (3/130): If a person writes a letter to a kaafir and wants to 
    include a greeting in it, he should write “Peace be upon those who follow 
    true guidance”, because that carries a comprehensive meaning. 

    The Prophet  (peace and 
    blessings of Allaah be upon him) used to use this phrase when he wrote to 
    the kings around him such as Chosroes, Caesar and al-Muqawqis. 
And Allaah knows best.