From: Rabbi Moshe Sofer
To: Rabbi Avraham
A woman who had relations while possibly a niddah (based on a kesem) began counting her seven clean days without waiting the requisite five days for politas shichvas zera. The Chasam Sofer rules that since the case involves only a kesem, which is a rabbinic prohibition, the clean days counted are valid bedi'eved. Regarding teshuvah, both husband and wife require atonement, but since only a rabbinic prohibition was violated, the penance need not exceed the standard prescribed by the Rema at the end of YD 185.
Source: Shu"t Chasam Sofer YD §188