Responsa for Bekhorot 47:12
איבעיא להו
clinging to the animals that had given birth previously and the females clinging to those which were now giving birth for the first time, he need not fear that perhaps the offspring of one came to the other.<span class="x" onmousemove="('comment',' So as to impose a restriction and make them questionable firstlings, fearing lest the males belong to the animals which are giving birth for the first time. The reason why R. Simeon the son of Gamaliel speaks of entering at night etc. and does not state simply that if one entered his herd and saw males clinging etc., is in order to inform us of a striking thing, that although their birth certainly took place in the night when the dams did not as yet recognize their offspring and were, therefore, liable to make a mistake, nevertheless we do not fear lest the offspring of the one came to the other.');"><sup>7</sup></span>
Teshuvot Maharam
A. The fact that a cow yields milk is no proof that she has already given birth to a calf, for it sometimes happens, though very rarely, that a cow yields milk even though she has not as yet given birth.
SOURCES: Cr. 312; L. 187; Orhot Hayyim p. 21. Cf. Moses, Minz, Responsa 34; ibid. 35; Terumat Hadeshen 271; Isserlein, Pesakim 129; ibid. 167.