Part 9 · Chapter 23

Now, three times are found in the matter of the Ibur [conception], as will be explained below. The first is an Ibur of seven months, as we find with Moses, peace be upon him, and with Peretz and Zerach and their like. The second is an Ibur of nine months, which is most of the world. The third is an Ibur of twelve months, as mentioned in tractate Yevamot, page 80b: a case came before Rabba Tosfa'a of one who tarried twelve months, and he declared him fit.