From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer resolves two objections raised against the Kesef Mishneh's explanation of the Rambam's list of women subject to me'es le'es tumah. He demonstrates that the Kesef Mishneh had to cite the braisa from Perek Yotzei Dofen because it alone provides the basis for applying retroactive tumah to a yoledes who saw no blood, via the principle of metamei bifnim kabachutz combined with the absence of sensation during labor. The "other women" included by kal vachomer are the poletes shichvas zera and a woman who expels a kezayis of neveila, though a difficulty regarding Shammai's position vis-à-vis the poletes is left unresolved.
Source: Shu"t Chasam Sofer YD §151