From: Rabbi Moshe Sofer
To: Rabbi Avraham Ullmann
A she-goat's next offspring retains bechor status because the chezkas mevakeres (presumption of no prior birth) is not overturned by a non-Jew's casual remark about a prior miscarriage. Even if the shepherd's statement were credited, the nature of what was miscarried is unknown, leaving the status doubtful. Furthermore, one may not retroactively question the shepherd, as such extracted testimony does not qualify as mesiach lefi tumo.
Source: Shu"t Chasam Sofer YD §308