Praise be to Allaah.

this issue further: 

1 – It is haraam to marry a woman who is in ‘iddah following the end of a marriage (by divorce or death) to another man, because Allaah says (interpretation of the meaning): 

“And do not consummate the marriage until the term prescribed is fulfilled”

[al-Baqarah 2:235]

The wisdom behind that is that there is the possibility that she might be pregnant [from the first husband], and it would result in the “waters” (sperm) being mixed and the lineage being confused (i.e., if he were to marry her before the ‘iddah was over).

2 – It is haraam to marry a zaaniyah (a woman who has committed fornication or adultery) if she is known to have  done that, until she repents and completes her ‘iddah, because Allaah says (interpretation of the meaning): 

“and the adulteress –fornicatress, none marries her except an adulterer — fornicater or a Mushrik]. Such a thing is forbidden to the believers (of Islamic Monotheism)”

[al-Nisaa’ 24:3]

3 – It is forbidden for a man to marry a woman whom he has divorced (talaaq) three times until she has been married to another man in a valid marriage which has been consummated, because Allaah says (interpretation of the meaning): 

“The divorce is twice… And if he has divorced her (the third time), then she is not lawful unto him thereafter until she has married another husband…”

[al-Baqarah 2:229-230]

4 – It is forbidden to marry a woman who is in ihraam [for Hajj or ‘Umrah] until she has exited the state of ihraam. 

5 – It is forbidden to be married to two sisters at the same time, because Allaah says (interpretation of the meaning): 

“Forbidden to you (for marriage) are: … two sisters in wedlock at the same time”

[al-Nisaa’ 4:23]

It is also forbidden to be married to a woman and her paternal aunt or a woman and her maternal aunt at the same time, because the Prophet (peace and blessings of Allaah be upon him) said: “Do not be married to a woman and her paternal aunt or a woman and her maternal aunt at the same time.” (Agreed upon). And the Prophet (peace and blessings of Allaah be upon him) explained the reason behind that when he said, “If you do that then you have broken the ties of kinship.” That is because of the jealousy that exists between co-wives, and if one of them is related to the other, the ties of kinship between them will be cut. But if a woman is divorced and her ‘iddah is over, then her sister, paternal aunt and maternal aunt become permissible for marriage (to that man), because the reason for the prohibition has ceased to exist. 

6 – It is not permissible to be married to more than four women at one time, because Allaah says (interpretation of the meaning): 

“then marry (other) women of your choice, two or three, or four”

[al-Nisaa’ 4:3]

The Prophet (peace and blessings of Allaah be upon him) commanded those who were married to more than four women to divorce some of them when they entered Islam. 

And Allaah knows best.