A:
Having sexual intercourse with one's wife during her period is Haram (prohibited), based on Allah's Saying (Exalted be He), (Part No. 19; Page No. 273)
They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have a sexual intercourse with his wife while she is having her menses), therefore keep away from women during menses and go not unto them till they are purified (from menses and have taken a bath). And when they have purified themselves, then go in unto them as Allâh has ordained for you (go in unto them in any manner as long as it is in their vagina).
The Prophet (peace be upon him) forbade this and the Ummah (nation) agree unanimously on its prohibition, so you have to seek forgiveness from Allah and turn to him in repentance, for having had sexual intercourse with your menstruating wife, particularly that she brought it to your attention that she was in her menses. You should have been cautious before doing this with her. You should also give a Dinar or half a Dinar in charity as a Kaffarah (expiation) for what you have done.
Ahmad and Ahl-ul-Sunan (authors of Hadith compilations classified by jurisprudential themes) reported
with a good Isnad (chain of narrators), on the authority of
Ibn `Abbas (may Allah be pleased with both of them) that
The Prophet imposed on whoever had sexual intercourse with his wife in her menses the payment of a Dinar or half a Dinar as charity.
Whichever you pay (i.e. a Dinar or half a Dinar), it will be sufficient. The Dinar is equivalent to 4/7th of one Saudi pound, so if the value of the Saudi pound was seventy Riyals, for example, then the amount you would be obliged to give in charity to the poor would be twenty Riyals, or forty Riyals. May Allah grant us success. May peace and blessings be upon our Prophet, his family and Companions.