Having sexual intercourse with one’s menstruating wife is a grave sin and disobedience for Almighty Allah prohibited this. Thus, you have to repent sincerely and ask for Allah’s forgiveness, and it is also recommended to pay the worth of a dinar or half a dinar of gold as expiation. A dinar is equal to 4.25 grams.

Dr.Hatemal-Haj, associate professor of Fiqh at Shari`ah Academy of America and Islamic University of Minnesota, states:

During her menses you may have all forms of sex with your wife except intercourse, for Almighty Allah says, (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-Baqarah2: 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 allowed because both the woman and the man still need to be emotionally nurtured during those days.
Islam provides the path of moderation regarding this issue, by protecting the well-being of both parties, as it is known that intercourse at this time would be unhealthy. At the same time, Islam recognized their need of the husband and wife to continue their affectionate relationship. It also emphasizes that the woman is not spiritually unclean. Having intercourse with a menstruating woman is an enormity. However, if you do this, you must repent, and the doors of repentance are always open. The need to expiate is a controversial matter, but I believe it is mandatory to give the value of a dinar or half a dinar of gold in charity.