Part 11 · Chapter 21

And afterward there is a third period, about which our sages said (Yevamot 41a) concerning the matter of the first three months, when the fetus becomes recognizable, as our sages said in the mishna, that the Yevama should neither perform Chalitza nor be taken in levirate marriage until she has had three months, when the pregnancy becomes recognizable. And the matter is that after the forty days of the forming of the child, when the filling of SAG entered there, which shapes the form of the child, another forty-six sparks of the filling of AV enter it over another forty-six days.