From: Rabbi Moshe Sofer
To: Rabbi David
The Chasam Sofer rules that if the woman performed a bedikah on either the first or seventh day of the shivah nekiim (but not both), the immersion and subsequent cohabitation are valid bedi'eved, relying on the overwhelming majority of Rishonim who follow Rav's lenient position. However, if she only performed a bedikah on a middle day, the situation is problematic even bedi'eved, since the Razah, Ra'ah, and Rashba are stringent on middle-day bedikah. He develops an original framework explaining that a bedikah on day one or seven serves as the external manifestation of intent to begin or conclude the count, while a middle-day bedikah carries no such demonstrative significance.
Source: Shu"t Chasam Sofer YD §178