From: Rabbi Moshe Sofer
To: Rabbi David Katz
After childbirth, Ashkenazi custom requires a woman to count seven clean days and immerse in a mikvah even during the dam tohar period, though this obligation is based purely on custom rather than Torah or Rabbinic law. The Chasam Sofer rules that a berachah should be recited on this immersion, arguing that custom creates a binding prohibition through the principle of 'al titosh toras imecha,' giving the resulting tevilah the force of a legally required act that warrants the full vetzivanu formula.
Source: Shu"t Chasam Sofer YD §191