Commentary for Niddah 48:28

ולד הוא אלא שיש לו כנפים

was necessary: As it might have been presumed that anyone [born within] less than two months [before the conclusion of the normal conception] can survive,<span class="x" onmousemove="('comment',' Sc. as in the case of man and large cattle one born at seven months after conception (two months before the normal period of nine months) is viable (though one born at eight months is not viable) so also in the case of small cattle (though one born at four months is not viable) one born at three months after conception (also two months before the normal period of five months) is viable. ');">²⁴</span>

Rashi on Niddah