Sforno on Exodus 22:29/ספורנו על שמות כ״ב:כ״ט

1כן תעשה לשורך לצאנך, that you shall give to Me the firstborn which is not a prematurely aborted fetus. Our sages have said (Shabbat 135) that “a human baby which remains alive for 30 days is considered viable, not in the category of נפל, prematurely aborted. In the case of animals the parallel period is eight days.”