Reference for Chullin 135:22

תנינא

Now this is so because the head [of the nine-months child] had emerged dead, but had it emerged alive then the child that was born after this would not be considered a firstborn, even for the right of inheritance!<span class="x" onmousemove="('comment',' Thus establishing the principle that with the emergence of the head the child is deemed born.');">¹¹</span>

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