Praise be to Allah the Lord of the Worlds.
Scholars are agreed that it is forbidden to divorce wife during her menstrual period because Allah the Almighty Says (What means): "O Prophet! When ye do divorce women, divorce them at their prescribed periods, and count (accurately), their prescribed periods." {At-Talaq, 1}.They considered this as innovative divorce for which the husband is considered sinful. This is attested to by the following Hadith: Ibn `Umar bin Al-Khattab divorced his wife during her menses. Allah's Messenger (PBUH) ordered him to take her back till she became clean, and when she got another period while she was with him, she should wait till she became clean again and only then, if he wanted to divorce her, he could do so before having sexual relations with her. And that is the period Allah Has Fixed for divorcing women." {Agreed upon}.
According to the Imams of the fours legal schools, divorcing wife during menstrual period is effective although it is considered innovative divorce. The evidence on this is the Prophet (PBUH) ordered Ibn Omar to take his wife back and this happens after divorce takes place and since the narrator of the above Hadith said that one divorce took place. According to Muslim, "'Ubaidullah reported: I said to Nafi': What became of that divorce (pronounced within 'Idda)? He said: It was as one which she counted."
Ibn Qhoddamah stated: "If he divorced her during the menstrual period (innovative divorce) or had sexual relation with her while clean then he is considered sinful and the divorce is effective as stated by the majority of the scholars." {Al-Moghni, vol.7/pp.279}. And Allah the Almighty Knows Best.