Halakhah su Levitico 12:1

We offer two explanations for why the Torah, according to the Ramban, granted such permission to our ancestors. One possibility is that this permission demonstrates the principle of Yalta (Chullin 109b) that for everything forbidden there is a permitted counterpart. The examples he offers include cheilev (fat) of a chayah (a non-domesticated animal) which is the kosher counterpart of the forbidden fat (cheilev) of a beheimah (domesticated animal), a part of the permitted shivuta fish (which tastes like pig) which is the counterpart to non-kosher pig and dam tohar (see Vayikra 12:1-8) which is the counterpoint to dam niddah.

The Gemara (Shabbat 130-134) teaches that we perform the milah even on Shabbat if that day is the eighth day of the baby's life. The Gemara (Shabbat 135) notes that this applies only to a baby born in a manner where the mother is rendered ritually impure (as described in Vayikra 12:1-8). Thus, we do not circumcise a baby that was born by caesarean section on Shabbat (see Shulchan Aruch Y.D. 266:10), as a mother becomes ritually impure at birth only upon a "conventional" birth.

The Gemara (Chagigah 16a) discusses a strange situation in which a woman becomes pregnant due to visiting a bathouse that previously was used by a man. Commenting on this Gemara, Rabbeinu Channaneil writes, "This is a miraculous act, and a woman does not become ritually impure upon this type of conception because it does not meet the specifications of the Pasuk [Vayikra 12:1] 'when a woman conceives and gives birth.'" Thus, according to Rabbeinu Channaneil, we may not circumcise the child conceived by bathhouse insemination on Shabbat.