Midrash su Levitico 15:31

1) (Vayikra 12:2) ("Speak to the children of Israel, saying: If a woman give forth seed and bear a male, she shall be unclean seven days; as the days of her menstrual flow shall she be unclean.") This applies to "the children of Israel" and not to gentiles (even if she converted within the days of uncleanliness). "the children of Israel": This tells me only of the children of Israel. Whence do I derive (for inclusion) proselytes and maidservants, whether freed or not freed? From (the generic) "a woman." Variantly: Because it is written (Vayikra 15:31): "And you shall separate the children of Israel from their uncleanliness, that they not die in their uncleanliness by defiling My sanctuary which is in their midst," I might think, whether from its midst or from its back; it is, therefore, written in respect to a yoledeth (a woman after childbirth) (Vayikra 12:4): "And into the sanctuary she shall not come."

(Fol. 18b) Our Rabbis were taught: (Lev. 15, 31) Thus shall ye separate the children of Israel from their uncleanness. R. Joshua said: "From this you may derive a warning that the children of Israel should separate themselves from their wives near the period of menstruation." And for how long? Said Raba: "For twelve hours" [prior to open period]. R. Jochanan said in the name of R. Simon b. Jochai: "He who does not separate himself from his wife at the said period, even if his children are equal to the sons of Aaron, they will die, for it is written (Ib.) Then ye shall separate the children of Israel from their uncleanness, … and of her that is sick from her impurity, and immediately follows the mention of the death of Aaron's children." R. Chiya b. Acha said in the name of R. Jochanan "He who does separate himself for that period will be rewarded with male children, as it is written (Ib. 11, 47) To make a distinction between the unclean and the clean, and immediately follows (Ib. 12, 2) If a woman … and born a male child." R. Joshua b. Levi added: "He will be rewarded with sons who will be fit to decide law questions, as it is written (Ib. 10, 10-11) So that ye may be able to distinguish … to teach." R. Chiya b. Abba said in the name of R. Jochanan: "Whoever pronounces Habdala over a cup of wine on the eve of Sabbath, will be rewarded with male children, as it is written (Ib. 10, 10) To distinguish, and also (Ib. 11, 17) To make a distinction between, etc., and immediately follows (Ib. 12, 2) If a woman … and born a male child."

2) I might think that a yoledeth, (whose tumah is) of lesser stringency, confers tumah (only) from its (the sanctuary's) midst, and all others who are tamei, both from its midst and from its back. It is, therefore, written (Vayikra 15:31) "the children of Israel." The children of Israel are being compared (in context) to a yoledeth. Just as a yoledeth confers tumah only from its midst, so, all (the children of Israel) confer tumah only from its midst.