Having sexual intercourse with one’s menstruating wife is a grave sin and disobedience for what Allah prohibited. Thus, you have to repent sincerely and ask for Allah’s forgiveness, as well as it is recommended for you to pay the worth of a dinar or half a dinar of gold as expiation. The dinar is equal to 4.25 grams.
In his response to question of this nature, Dr. Hatemal-Haj, stated:
“You may have all forms of sex with the wife except for intercourse, for Allah said,
“They ask thee concerning women’s courses. Say: it is harm: So keep away from women in their courses, and do not approach them until they are clean. But when they have purified themselves, ye may approach them in any manner, time, or place ordained for you by Allah. For Allah loves those who turn to Him constantly and He loves those who keep themselves pure and clean.” (Al-Baqarah 2:222)
The Prophet (peace and blessings be upon him) explained what is meant by keeping away from women by saying, “Do everything except for intercourse.” (Muslim) This is because the woman as well as the man will continue to need the emotional nurture of their relationship during those days.
Islam provides the path of moderation regarding this issue, protecting the well being of both parties, as it is known that intercourse here would be unhealthy, while recognizing their need to continue their affectionate relationship and emphasizing that the woman is not spiritually unclean.
Having intercourse with a menstruating woman is an enormity. However, if you fell into it, repent, and Allah’s doors are always open to those who do. As for the need to expiate, it is controversial, and I believe it is mandatory to give the worth of dinar or half a dinar of gold in charity.”