From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer defends the Shach's ruling that a bride may not immerse during the day on her seventh clean day but may on the eighth. He argues that after the chuppah, post-yichud seclusion creates a halachic presumption of he'arah, making the shema tir'eh v'sistir concern fully applicable on the seventh day; on the eighth day, however, serach bitah does not apply because a daughter would never presume that he'arah occurred during the day when full consummation is universally expected to take place only at nightfall.
Source: Shu"t Chasam Sofer YD §189