《礼记·郊特牲》
这是因为尊重真诚之心的缘故。所以天子不吃怀孕的牲畜，祭祀上帝也不用怀孕的牲畜。
故天子牲孕弗食也，祭帝弗用也。
This was sacrificing to the sincerity of truth. Hence the son of Heaven did not offer pregnant animals, and did not use them for the sacrifices to God.