She married the one who committed zina with her after she gave birth and the nifaas bleeding stopped
Question

                There is a woman who got pregnant from a man, then she gave birth and when the bleeding stopped she married this person, before the end of forty days. Is their marriage contract valid or not?.
Praise be to Allah.Firstly: 
What this woman has to do 
    is repent to Allaah from what she has done, because zina is a grave sin, a 
    serious crime and a great evil. The one who commits zina is warned of a 
    severe punishment in this world and in the Hereafter, but the one who 
    repents and mends his ways, Allaah will accept his repentance and turn his 
    bad deeds into good deeds, as 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] 
“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]
She must also conceal 
    herself with the concealment of Allaah and not tell anyone about that. The 
    Prophet  (peace and blessings of Allaah be upon him) said: “Avoid these 
    filthy things that Allaah has forbidden. Whoever has done any of them, let 
    him conceal himself with the concealment of Allaah.”
Narrated by al-Bayhaqi and 
    classed as saheeh by al-Albaani in Saheeh al-Jaami’ (149). 
Secondly:  
It is not permissible for 
    the zaani to marry the zaaniyah until after they have repented, because 
    Allaah says (interpretation of the meaning): 
“The adulterer — 
    fornicator marries not but an adulteress — fornicatress or a Mushrikah; and 
    the adulteress –fornicatress, none marries her except an adulterer — 
    fornicater or a Mushrik [and that means that the man who agrees to marry 
    (have a sexual relation with) a Mushrikah (female polytheist, pagan or 
    idolatress) or a prostitute, then surely, he is either an adulterer — 
    fornicator, or a Mushrik (polytheist, pagan or idolater). And the woman who 
    agrees to marry (have a sexual relation with) a Mushrik (polytheist, pagan 
    or idolater) or an adulterer — fornicator, then she is either a prostitute 
    or a Mushrikah (female polytheist, pagan, or idolatress)]. Such a thing is 
    forbidden to the believers (of Islamic Monotheism)”
[al-Noor 24:3]. 
If they had repented to 
    Allaah before getting married, then their marriage is valid. But if they did 
    the marriage contract before repenting, then the marriage is not valid. This 
    has been discussed in the answer to question no.
    85335. 
With regard to the 
    marriage  contract being done after the bleeding stopped but before forty 
    days had passed, this does not affect the validity of the contract, because 
    what she should do – according to many scholars – is wait until her ‘iddah 
    following zina ends, then do the marriage if she wishes, even if that is 
    within the nifaas period. Her ‘iddah ended when she gave birth. 
The Standing Committee for 
    Issuing Fatwas was asked: If a man commits zina with a woman then they get 
    married, and after four months he repents to Allaah, is the marriage valid? 
They replied: It is not 
    permissible to marry a zaaniyah and the marriage contract with her is not 
    valid until she repents and her ‘iddah ends. 
End quote from Fataawa 
    al-Lajnah al-Daa’imah (18/383). 
To sum up: If they had 
    repented from zina before doing the contract, then the marriage is valid. If 
    they had not repented, then the marriage is not valid and they have to 
    repeat the marriage contract after repenting. We ask Allaah to accept their 
    repentance. 
And Allaah knows best.