From: Rabbi Moshe Sofer
To: Rabbi Ephraim
A God-fearing Torah student had relations with his wife twice in one night, not knowing she had become a niddah; upon rising, the sheet was found saturated with blood. The Chasam Sofer analyzes whether each spouse is a meizid, shogeg, or ones, concluding the husband is a complete ones while the wife bears the greater guilt for failing to inform him. He prescribes forty non-consecutive fast days for the husband, abstention from meat and wine for the wife, charity equal to a chatas goat given to Torah scholars, increased Talmud study, daily confession, and annual observance of the date as a personal day of teshuvah; he firmly rejects the notion that the wife's being under twenty exempts her from Heavenly punishment.
Source: Shu"t Chasam Sofer YD §155