From: Rabbi Moshe Sofer
To: Rabbi David Katz
A woman who immersed during yemei tohar and saw blood again asked whether she could begin counting shivah nekiyim after only four days rather than the standard five. The Chasam Sofer rules she must wait the full five days, refuting the kal vachomer from dam besulim by demonstrating that dam tohar and dam besulim differ in their peru u'revu implications, and that any leniency would be offset by the reinstated obligation to separate on the night of the forty-first or eighty-first day.
Source: Shu"t Chasam Sofer YD §157