What is the ruling on marrying a Jewish or Christian woman who is pregnant from a previous relationship?

Firstly: 

In order for marriage to a Jewish or Christian woman to be valid, it is stipulated that she should be chaste, because Allah, may He be exalted, says (interpretation of the meaning):

“(Lawful to you in marriage) are chaste women from the believers and chaste women from those who were given the Scripture (Jews and Christians) before your time, when you have given their due Mahr (bridal money given by the husband to his wife at the time of marriage), desiring chastity (i.e. taking them in legal wedlock) not committing illegal sexual intercourse, nor taking them as girl-friends”

[al-Maa’idah 5:5]. 

What is meant by “chaste women” in this verse is free (i.e., not slave) and chaste women from among the People of the Book. It is permissible for a Muslim man to marry them. 

But if the woman is not chaste, then it is not permissible to marry her, regardless of whether she is Muslim or Jewish or Christian, until she repents. 

Shaykh ‘Abd ar-Rahmaan as-Sa ‘di (may Allah have mercy on him) said: As for immoral women who do not refrain from zina (unlawful sexual relationships), it is not permissible to marry them, whether they are Muslim or Jewish or Christian, until they repent, because Allah, may He be exalted, says (interpretation of the meaning): “The adulterer marries not but an adulteress or a Mushrikah…” [an-Noor 24:3].

End quote from the Tafseer of Shaykh as-Sa‘di (p. 221) 

Based on that, it is not permissible for you to marry the woman mentioned, because the condition of chastity is not fulfilled in her case. 

For more information, please see the answer to question no. 33656. 

Secondly: 

If a woman is pregnant from marriage or an illicit relationship, it is not permissible to marry her until her ‘iddah ends. The ‘iddah ends when her pregnancy ends, based on the report narrated by Abu Dawood (2185) from Ruwayfi‘ ibn Thaabit al-Ansaari (may Allah be pleased with him) who said: I heard the Prophet (blessings and peace of Allah be upon him) say: “It is not permissible for any man who believes in Allah and the Last Day to pour his water on the crop of another man (i.e., have intercourse with a pregnant woman).” Classed as hasan by Shaykh al-Albaani in Saheeh Sunan Abi Dawood

Based on that, if that woman repents from zina (illicit sexual relationship) and becomes chaste, then it is permissible for you to marry her after the end of her ‘iddah, which is when she gives birth. The same conditions are stipulated for marriage to a Christian woman as are stipulated for marriage to a Muslim woman, namely the permission of her guardian and the presence of witnesses, as has been explained in the answer to question no. 159297. 

What has been explained above regarding this issue has to do with the shar‘i ruling. However with regard to advice, our advice is that a Muslim should not go ahead with such a marriage, firstly because there is no guarantee that this woman will not go back to her previous way of life, having illicit relationships. Moreover, marriage to a Christian or Jewish woman will have an impact on the religion and character of the children and of the husband in most cases. So refraining from such a marriage is preferable and safer. 

Since even in marriages to chaste Jewish and Christian women there have been real-life negative consequences and problems, which would make a wise man hesitate and ask himself a hundred and one times before going ahead with such a marriage, then how will it be if the situation is as you have described? In that case you will have combined two troublesome matters. We ask Allah to keep you safe and sound. 

And Allah knows best.