Yoreh Deah Siman 15 Seif 2

 

ב בְּהֵמָה שֶיָּלְדָה, אִם יָדוּעַ שֶׁכָּלוּ לוֹ חֳדָשָיו, דְּהַיְנוּ ט’ חֳדָשִׁים לְגַסָה, וְה’ לְדַקָּה, מֻתָּר מִיָּד בַּיּוֹם שֶׁנּוֹלָד, וְלֹא חַיְשִׁינָן שֶׁמָּא נִתְרַסְקוּ (פי’ נִכְתְּשׁוּ וְנִכְתְּתוּ) אֵבָרָיו מֵחֶבְלֵי הַלֵּדָה; וְאִם אֵין יָדוּעַ שֶׁכָּלוּ לוֹ חֳדָשָיו, אָסוּר מִשּׁוּם סָפֵק נֵפֶל עַד תְּחִלַּת לֵיל שְׁמִינִי.

 

Seif 2:

When an animal is born, if you know it was born after a full-term pregnancy, which would be 9 months for large cattle and 5 months for small animals (such as sheep and goats), it is permitted to slaughter it immediately on the day it is born, since we are not worried that its limbs were ruined from the difficulty of childbirth.  However, if it is unknown whether it was from a full-term pregnancy, it is forbidden until the eighth night, inasmuch as it is possibly considered to be a miscarriage until the eighth day.